these are two basically identical cameras at their core, the major difference being the design layout of the buttons. if you already own canon equipment, go with the t1i, if you already shoot nikon, the d5000 is the obvious choice. it all comes down to personal preference, really. half the people on this forum will tell you that canon is the better brand, the other will back nikon.
with that being said, im a nikon shooter, but if i didnt already have a camera and was looking for something in that entry level market, i would probably go with the t1i because the video quality and control is going to be better (the only solid difference between the two cameras).
good luck with your purchase.