dark teas can stain your teeth, like coffee does. brushing them afterwards should help with it, though.
I think I once read that green tea is good for your teath, just stains them. It might be a bit acidic but I'm not sure. If I can find my pH paper I'll test some and post the results, not sure where it is though so I may never find it.
its the tanin in tea which stains your teeth, but i've heard lemon juice or bicarb of soda are good for getting teeth white again, not sure if they work tho so don't take my word for it!