For what it is worth, I know the poster above was being sarcastic, but the Pentax line of cameras has a decent amount of stuff to offer and should not be casually overlooked.
And, for what it is worth, I believe you will find that all Pentax lenses are compatable with the modern Pentax dSLR. Of course, you might need a $30 adaptor, but I use atleast one M42 screwmount lense with my Pentax *ist Ds. However, I do not use any Nikon or Canon lenses. I had one Canon lense, given to me by a relative. I gave it to a friend who uses a Canon Rebel, and he was unable to use it, as it was "too old" of a lense, even though it wasn't very old at all. Of course, he takes better pictures than I do, but it's a talent thing.
If you do not know enough to know that not all lenses are compatable with all SLRs, then I do NOT suggest you spend the money on the newest models. Buy something two years old and learn on it. It'll be 6mp for certain and will be just fine for you to learn with. Don't buy the fighter jet if you don't have your pilots license, don't buy the Porsche when you've only driven a gokart, and don't buy an XTi when you don't know anything about SLR's. Infact, I'd advise you to spend $100 on a film SLR and $20 on any old book about photography first.