The Google tool's been used for over a year now, both for all-hands (although that's a bit more recent) and for submitting questions to Tech Talks (particular from remote sites) and major visitor talks such as the Presidential candidates (for the latter, the top-ranked were passed on to the interviewer, rather than being asked by proxies at the mike). It also allowed you to comment on a question, although those were only visible if a user chose to make them so.

One problem with it, at least for the version being used when I was last there, was that there was a definite advantage to submitting your question early on, since more people would then see it and have a chance to vote on it. If you submitted something right before (or during) a presentation, you probably wouldn't get enough votes to break into the top tier. I did suggest to the person working on the next version that perhaps something could be added so that if you'd voted, you'd get a reminder email N minutes before the event's start to go back and vote for any newer questions that'd been submitted since your vote.