I would get the Pentax K-x. While both are excellent cameras, the Pentax K-x offers these advantages:
*Pentax K-x has higher ISO setting of 3200, 6400 (100-12800 with boost) while the Canon XSi only goes to 1600 ISO. This can make a big difference when shooting in low light situations. The first website link below is to a side by side listing of the features and specs of the 2 cameras. This listing page also has links to reviews with sample images from the camera.
*The Pentax has a higher rated image sensor. The 2nd website link below is to DxO Labs to a page
showing the sensor rating for both cameras.
*Image Stabilization, it is in the body and it appears that it being in the camera body means it can help with a great variety of lenses.
*The Pentax has HD video which could be a nice extra.
*The Pentax costs less, here is a quick price comparison of prices from B&H Photo:
EOS Rebel XSi (1000D) 12MP DSLR Digital Camera Kit w/18-55mm IS Lens - $560
Pentax K-x 12MP DSLR with 18-55mm Zoom (has HD Video also) - $514
I also added a few websites that are good to shop for deals and offer cutomer reviews.
Hope this helps.
Mark
marksablow.com