Basically, if you're getting a modern DSLR within a given price range, they are all going to perform pretty much the same.
Some models might have extra features which you prefer (worth keeping an eye on). For instance, I have a Canon 60D - I liked the idea of the flip out screen and it's pretty good recording ability.
Image quality is going to be stinkingly good on most modern DSLRs, I'm sure there are some dud models out there, but they'll be few and far between. People will review and compare image quality, but the nuances in difference are going to be tiny and the lenses you use are going to be the biggest influence. Then, regardless of the big brands, they all make stinkingly good lenses!
If you were an out and out pro, you might notice the difference in image quality more, but even then nobody is going to look at a printed picture and go "aha! That was a Canon/Nikon/Pentax/etc." .
One thing I will add, and I'm not sure if anyone else has said yet. But go in and try the cameras out, see how the weight feels, see how the build feels and see how it sits in your hands. Even though DSLRs all look pretty much the same, some are surprisingly bigger than others and some are small enough to feel like toys. The button placement is also something to take into consideration when holding onto one in a shop, some will just feel more intuitive than others.
Edit - crap, beaten to the post with the last points!