Don't have a Nikon, but for >30 second exposures, check for "Bulb" or
similar in Manual mode, and yes, you either have to keep your finger on
the button or get a cable release that will lock in the "open"
position.

And "metering" won't work, either - you have to guestimate and
testimate your very long exposure shots.

My Canon Timer remote was easily the best $135 I've spent related to
photogarphy.
Nikon likely makes a similar remote...  With it, I can not only trip the
shutter (and prefocus if preferred), but I can tell it how many frames to
take...at what interval of time...and after waiting for a time I specify--  
and how long to hold the shutter open (without having to hold it myself) up
to many hours(!) if necessary--though I would only suggest using film for
such an exposure.   This is great for group shots where you are part of the
group, for example, because it means you can tell it to take 5 or 6 (or 99)
shots without having to walk back over the the the camera each time.  It's
also got an illuminated LCD, which makes an awful lot of sense considering
that you'd often use it in dark settings.

Settings/controls include:
-Time until first frame
-Number of frames
-Time between each frame (interval)
-Shutter open time (up to 99 hours+)
-Start/Stop
-Shutter with half/full press
-Lock shutter switch (so you don't have to actually HOLD it down during
extremely long exposures)
-Adjustment wheel
-Ilumination button

Not bad for the $135.  Surely Nikon has something simlar.

About your first question...  It's normal to have a 30 second limit on even
high-end SLR/DSLRs for auto, or dialed-in exposure.  Beyond that, you use
"bulb" setting, which is basically holding it down, as you said.  -But of
course you'd never want to actually do that by hand.  You need that remote
shutter release.

-Mark²

Paul:
See Pg. 5,  at the bottom!
Bob

Reciprocity failure takes over at long and short exposures for film.  I've
found it to be similarly true for digital.

Have you tried the multiple exposure route?  I used to do that with my F100
and film if I didn't have a cable release.

The meter shows "Lo" (in aperture priority, at least), or it gives an
indication but flashes (in manual) below some light level. I don't
think it's about the 30s limit at all, at higher ISOs it'll flash at
you (or say "Lo") at shorter exposures. But yes, the thing can't do
more than 30s timed exposures; most SLRs can't, but I am sure some can
and it would obviously be completely trivial to implement (eg by
entering how long the exposure should be). Given how easy it is to
implement, I infer that Nikon simply wants to sell us the remote. So I
got the "ADIDT" cheapo one. Works perfectly (except when it is plugged
in, the camera goes to sleep and you wake it up by a shutter release
half-press: it then takes a shot instead of just waking up, extremely
irritating when mirror lockup is used). It can be locked pressed (for
arbitrarily long exposures).

Of course, it is also hugely overpriced for what it is, just less so
than Nikon's remote.

Forgot to say in my previous response: the on-off switch has on/off
positions and a spring-loaded third position which lights the top LCD
(it's indicated by a lamp).

Hello there.

The same was true of many film SLRs. The 30 second maximum could be extended
with accessories.
I've a Canon T90 with a Command Back which would offer exposures longer than
30 seconds.

Regards,  Ian.

Doesn't Nikon Capture let you do this? Of course if it does you would
need a laptop in the field which would be a lot more expensive than a
remote! ;)

Most high end SLR cameras meter down to about EV 0 or -1

EV -1 comes to ISO 100 f/4 30 sec.  (Not accounting for reciprocity
failure in film).

Pretty reasonable.

That it won't give an exposure time for longer is most likely because
there simply is not enough light to measure and a display that is s/w
limited to 30 sec and not beyond.

Cheers,
Alan

... I neglected to add that EV -1 corresponds to a "landscape lit by a
full moon".