Between the Nikon D3100 and the Canon T1i, I'd personally go with the Nikon since I prefer smaller bodies. I also find that the lower-end consumer glass Nikon offers is better than Canon's entry level offerings. At the pro-grade levels I don't see much in difference either way from either company.
That being said, I'll throw my 2 cents in about Pentax.
1. The bodies themselves feel great in hands (either the K-x or K-r, major rub on K-x is it lacks autofocus indicators but I don't mind too much)
2. Same sensor as the D3100 meaning comparable image quality at least from the body perspective (though RAW processing is a bit worse if I recall correctly)
3. K-r is rated to have one of the best ISO (low light) capabilities currently available
4. Any lens, and I mean ANY lens ever made for the Pentax K-mount is compatible with ANY Pentax body dating back to 1977, and M42 lenses can be mounted easily with an adapter. This point is huge since you can pick up alot of great used lens extremely cheap, saving you some coin. Also any manual focus lenses that have an auto-aperture settings will work on aperture priority, while all manual focus lenses will auto-meter (not sure if Nikon implemented this into the D3100, but as far as I know this feature was absent in the slightly older D90)
Hate to muddle the decision more, but I honestly think Pentax is worth a look (not just because I own one - my dad has a Nikon set up and I think its just as good down to the glass, but I find knowing having full functionality on a lower end body to be a great selling point for the company).