Digital Anarchy is offering a free filter, Cartoon Bubble for Photoshop and Photoshop Elements, that creates word bubbles. It works nicely in combination with Cartoon filters.
You could also create word bubbles from scratch in After Effects, and Pro Juice has a new tutorial to show you how. In Thought Bubble (After Effects), Nick from Pro Juice uses masking techniques, motion tracking and shadows, to create "a thought bubble that floats alongside Arlo's head helping him decide what to do."
Note: Pro Juice recently did another tutorial on a more common task in Witness Protection, where they take you through 3 different ways to obscure and render a character unidentifiable in an interview situation.