How to implement a user profile attribute to indicate "Where did you hear about us"? I want to add a "Where did you hear about us?" checkbox list to user profiles. There will be 6 radio buttons and an "Other" textfield.

There's no need for a taxonomy vocabulary, which was my first thought. I also thought about using Webform, but that feels like overkill and I think I'd have to do more work to link the form to the user.

Really, I just want to view the user profile and say "Oh look, they found out about us from twitter ...". So what are good techniques/approaches to implement this?
This question does not say if it is about D6 or D7, but how about just a pull-down selection list (could be a checkbox as well if you prefer)? Similar to what you can see on this registration page, around the middle of the page (still a D6 site, D7 should be similar).

If it makes sense, I'd be happy to further enhance my answer to include the details of how I implemented that (piece of cake).

Note: please do not submit a real registration request if you checkout the link I added here ... unless you really want to sign up there (which requires prior approval ... ).