A lot of t-shirt slogans are actually pretty funny, enough to make me smile, it's just that now I've seen pretty much ALL OF THEM about 2151651565 times.
I usually buy shirts from www.topatoco.com - it's a webcomic site and a lot of the designs there are really cool even if you don't read the comics. Somebody linked a 'ninja's can't catch you if you're on fire' shirt up there, I bought that for a friend for his birthday.
I think the only one I've seen recently that stood out to me was a simple black shirt with plain white lettering which read "This is my favourite t-shirt" then underneath "(I really like it)". I don't know why, but I thought that was awesome.