Both D5000 and T1i are fine cameras. And of course the XSi are good too especially it cost less (minus the video feature).
If you are very serious about photography and like to learn more about it, starting with the XSi may not be a bad idea. At least you can save some money to buy some accessories such as flash, tripod or good lenses.
Personally, if I go with Nikon, I will get at least D90 just because of the fact that I like to buy the Nikon 50mm F/1.8 lens and D5000 cannot autofocus with that lens.
Did you go to a store and played around with both cameras? If not, you may want to do so. If you do not like the way the camera feel in your hand, that may take most of the fun out from your hobby. So try them out first and see if you like them. See how they feel in your hands, and see if you like one better than the other one in buttons layout.