At the Linden Lab Q&A today, following Joe 'Linden' Miller's podcast, Joe just announced that Mono had been deployed on the Beta Grid. It's now running on some of the sims there.
Congratulations to Babbage Linden and his team for rolling out Mono on the Beta Grid.
An official announcement will be coming shortly... in the mean time, download the SL Beta Viewer, hit the Beta Grid, and break the scripts there :)
[UPDATE: The official announcement was made by Babbage here].