We shipped an April CTP after MIX that was also go-live and we are planning to ship a June CTP of Atlas. The goal is to continue to ship CTP's every 6-8 weeks until we release.
We are currently committed to shipping an RTM supported version of Atlas with the "Orcas" release of the dot net framework and visual studio. HOWEVER, we are investigating pulling that date in considerably. That work is in the process of being finalized and
I hope to have information ot share later this summer.
Either way, the CTP drops are go-live, relatively stable, and deifnitely something you can use today.