[sdiy] LED's on negative rail??

[media at mail1.nai.net]

I'm working on a few things that run almost entirely off the postive rail.
Since LED's draw a significant amount of current (the light has to come
from somewhere), I'm thinking that I should run the LED's off the negative
rail to balance things out.  However, I've never seen that done.  Is there
any reason why??  Is it a noise issue??  As far as I can tell, the LED's
would "see" the same difference.

Any suggestions on what is the best way to drive an LED??  I've seen
circuits that use op-amps or FET, but wouldn't regular transistors work
just as well for less money??