Are the Dev nightly builds available to users or is it just the stable ones?
By default it is the stable only … the dev nightlies are being released every day and are not well tested. Because of that they happen to be broken occasionally (crash, render wrong or slow, etc). It is better to use the stable builds unless there is a reason to experiment with the nightlies :).
Best,
Blago.
Is this available now? If not, will it be available in 3.5?
Edit: Just seen this on the roadmap for 3.5 
woohoo!
It is supported in 3.5 beta and will be in the final 3.5 as well.
Best,
Blago.