Some things are blatantly obvious, yet I somehow still manage to miss them. If you have, too, then here it is.

You look up Ruby and Rails documentation from within TextMate by simply placing the caret inside a word and hitting ^H. It works using ri, so you need to make sure that your Rails documentation is available to ri.

Mine wasn’t, so I added the following line to ~/.gemrc:

rdoc: --all -S -N --ri

Then finally I did a sudo gem rdoc --all, and all was bliss.