Any on the list would be fine choices.   Frankly I believe the best
camera to learn on is the camera you have what ever it may be.  Learn to use
what you have and adjust to any restrictions it may have.

Any of those Nikons look fine for the purpose. I have the D40 with its
18-55mm kit lens and the D70s with its 18-70mm kit lens. Both are great. I
like the D40 for its light weight and more compact size; the D70s is the
more capable camera but with slightly older technology. The D40 requires the
newer SWM lenses for autofocus (the autofocus motor is in the lens) while
the D50, D70 and D70s will all use either SWM or somewhat older D and G type
lenses that require the autofocus motor to be in the camera body. This will
only be important to you if you plan on using the slightly older type
lenses. The D40 can use those lenses too, but only with manual focusing.

Apart from the lens issue, which may never be an issue for you at all, any
of the Nikons you mention should be good choices. The price of the D40 makes
it almost irresistible, but then the D50 and D70s are selling at closeout
prices that make them awfully attractive also.

I'm not really familiar with the Canon line so can't comment on that.

Neil

SA-

What are you trying to learn?  How to point-and-shoot?  With any one of
these you may be able to learn some things, but you might do just as
well taking an art class.

You should consider adding a lens.  One with a wider aperture, so you
can learn about depth-of-focus and available light photography.  Perhaps
a 50mm f1.4 would be good as a portrait lens on an APS-sensor digital.

How do these cameras compare for ease of manual adjustment?  How easy is
it to manually focus them?

I agree that these fine bodies may be equally adequate for learning
photography, but my personal choice would be a Pentax.  In my collection
of older film cameras plus some recent DSLRs, I much prefer the way the
Pentax handles.  It lends itself to manual adjustment, and a wide
variety of compatible AF and manual lenses are available on the used
market.

Fred

Learning photography is a broad subject. The real question is which
camera will give you the flexibility to experiment and learn how various
 settings (ISO, shutter speed, aperture, etc.)along with an appropriate
lens allow you to get the result you want without post processing.
Your choice of subjects (portrait, landscape, architecture, macro, etc.)
are going to place artistic demands on you independent of whatever
camera you choose. Great and bad photographs have been taken with
Brownie box cameras. Take the shot and see what framing and cropping
works to get a satisfying result. The beauty of digital is you can take
all the pics you want without the cost burden of film some of us had in
the past.

As one of those who has rapidly descended into old-fartdom I started
with my fathers WWII era Argus viewfinder and his darkroom equipment. I
moved on to a Yashica Electro 35, a Pentax K1000 and a Durst enlarger
all 38 years old and are now gathering dust and moths. All of these
taught me the mechanics of the camera and the end process.
My current armory includes a D70, a D200, a vault of lenses with a wish
list and Photoshop CS2.

I would suggest any of those you have listed with some of the best
bargains available on the D70 giving you financial room for the glass
you will desire. Then consider PhotoShop Elements for post processing
you might not need spend the money to move on to CS2/3.
Just get out there and shoot, ask for and ignore criticism and learn to
have an eye for what pleases you.
Good luck,
'Duck

If you want to really *learn* the ins and outs of photography, rather
than just take more-or-less automatic pictures right out of the box,
you should use manual focus and manual exposure adjustment. A fast
prime lens would be preferable to a (slow) zoom lens.

I would recommend the D40, because it's as good as the other ones in
manual control mode, and is cheaper. Use the savings to get something
like a 35mm f/2 AF-D lens. It won't autofocus on the D40, but you
don't want autofocus anyway if you're trying to learn photography.

Actually, if learning is your goal, you might want to consider a film
camera. However, the drawback there is the cost of film and
processing.

Of those listed I'd go with the D70, or alternatively a good, used Canon 20D
if you want to go for the Canon system. Both of these can be had for
relatively cheap. Either will do the job in great style, which one to buy is
largely a matter of feel and what you prefer in terms of ergonomics.

The other three cameras you listed are all castrated in some way. It
wouldn't matter so much if you just needed a camera for snapping pictures,
but for "learning the ins and outs of photography" it's another matter I
think. Neither of them has more than one control wheel, to really learn
photography you really, really want to get a good understanding of the
manual mode of operation (keeping in mind that automation will automatically
do the wrong thing a lot of times) and then you will want to have both
shutter and aperture control easily available. The D70 and 20D are the
minumum cameras that give easy access to both of these with separate wheels
so that you have one wheel for shutter speed, another wheel for aperture.
It's possible to do this on the D40/D50/XT too of course, but it's more
fiddly and this slows you down - to change aperture you have to keep a
separate button pressed while rotating the single control wheel. Hence the
D70 or 20D.

In addition to this drawback, the D50 also lacks a depth-of-field preview
button (no big deal perhaps) and a backlight for the top LCD (now THIS is a
big drawback once you are shooting in less than good light - if you ask me
Nikon saved ten cents in the wrong place there). The D40 lacks a built-in
focus motor, which is a total showstopper if you are ever going to buy
another lens. Nikon's selection of lenses with built-in focus motors is thin
in the extreme, and manual focusing is no fun at all on a digicam.

As far as the Canon XT goes there is nothing directly wrong with it except a
pretty crappy viewfinder and the fact that it is so small it can be
uncomfortable to handle if you have largeish hands. (And that it only has
one control wheel of course, but we've already been through that).