I've been thinking about this too. I've got some offensive humor shirt designs along with different categories, and I'm going to put it all on the same site. However, I've also thought about doing Christian shirts, since I grew up Baptist and know how Christians tend to think, so I will probably buy a separate domain name for that (else, they'd probably have a heart attack on seeing some of my more offensive designs!).
For a single website, you could go the subdomain route. For example, if your website is
www.example.com, and you were starting an anti-Bush category, you can set up this address too:
www.antibush.example.com. (Note: I don't own example.com and just made it up. Great domain name, though.)