Just this week I decided to start really teaching him to say it to give him an option rather than standing at my feet (or knees when I'm sitting) and screaming bloody murder at me. Since he's never been very interested in mimicking, I was kind of surprised he picked it up. I think it just took him a few times to realize it worked, and that was the key.
I was waiting to hear it a little more, and unprompted, before I decided if it was an accident or truly his first word. But yesterday he said it to me four times--three of them unprompted--and twice today--once to get out of bed and once to get out of the high chair after breakfast.
It sounds like this: ubf!