How to implement a user profile attribute to indicate "Where did you hear about us"?
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 ... ).