First off, Welcome to the forum.
I don't see any reason you couldn't learn on a D50. Depending on your ability to learn quickly, the D40 may not have enough features for you so you may out grow it too quickly. For the small price difference, you could also look at the D80, which as a beginner, you could really grow into.
They just announced a D40x which is 10mp... Not sure I get that move, didnt the D40 JUST come out a few months ago?
I have two coworkers that own a D50 and a D40. Both seem to like them very much. The woman that owns the D50 just moved to digital from film, so she was experienced with photography, but she is finding the D50 fits her needs quite well.
The only other thing I can add is do your research. I researched my first digital SLR, the D100, for about 9 months and owned it for 4 years. I just sold it to upgrade to the D200, which took me 6 months to decide between it and the D2X! If you have a local store that has the newer selections in stock, go check them out and see if the store will let you shoot with them a bit and see which works best for you!
Good luck! I hope you have deep pockets, because this gets addictive and expensive quickly!