I logged in on a bud's puter this weekend, and I forgot to hit log out. I assume I have a cookie on his machine that will log him in as me everytime he visits the site.

what should I do to protect my acct? change my password? ( I assume)

Yeah, changing the password is the only thing you can do.

I cann't log ya out from a given computer as the info is cookied on a given computer...

If you have any issues with that IM me and I will fix ya up.