Microsoft's most recent Roslyn description (according to a December 16 "C# Frequently Asked Questions" blog post) is "a replacement for our VB & C# compilers (and certain IDE pieces that leverage them)." The technology will allow developers "to reach inside the compiler iself," which according to Microsoft "will lead to far richer IDE (integrated development environment) and diagnostics being able to be developed at considerably reduced effort and cost."
Microsoft's last generally-available preview of Roslyn was out in September 2012.
"Yeah, we do feel a bit embarrassed about that," blogged Matt Gertz, VS Managed Languages Development Manager. "Yes, we’ve been a little quiet. No, there was nothing wrong going on here, things were actually going extremely well."
There's no word in today's post if there will be another Roslyn preview coming soon, or if the team will release the code as part of one of the expected Visual Studio 2013 updates in 2014. I've asked Microsoft execs; no word back so far.