chromatic aberation.  It is common with poor quality lenses,
especially in high contrast areas, such as the bright sky behind
leaves on a tree.   If you shoot in raw, it is possible to minimize it
to some degree, by using the sliders in photoshop raw filter.

Good quality lenese it is much less of a problem.  I guess another
alternative is avoid shooting in those circumstances that lead to
it.

There is too much blood in my alcohol system

Send this bloke $10 US. I did & it works bloody great in CS2. Also a great
prog for removing any colour really.

btw, it happens with L glass also...not just cheap lenses.

lens quality.... AND what sort of aperture range you are shooting at.

a lens performs at it's best in mid range apertures.... not the widest
and not the smallest.

when wide open, you have added abberations caused by the curvature of
the lense (towards the edges).

when stopped down to f22, you are then losing quality due to excessive
refraction around the aperture blades.

go to www.popphoto.com .... check out their lense reviews, you will
notice all lenses perform best at mid aperture settings!

and again... the better the lense... the less of a problem it is, though
impossible to erradicate entirely.

APO, ED, L , are all indicators (depending on manufacturer) of better
lenses specifically designed to minimise this effect.....

good luck.

those cheap and nasty T* lenses are terrible for it.

fortunately software/firmware can fix bad lenses

k