I would be a pain to carry around, but a bluetooth keyboard should work with your phone as well. The make some small bluetooth keyboards that are about the size of a candy bar phone as well.
They also make some which project a complete keyboard on the table before you, so you type on the table itself (i.e. you can use it anywhere). Unfortunately, it's really not ready for prime time, as it's little more than novelty value. :(
Oops! Midnige already covered that!
I've always values the keyboard cases by Logitech, though they are also plastic keyboards you can use which roll-up, so they're easier to transport (easier to use with phones than a full-size keyboard).