Josh has mentioned a free weekend for the engine to. Next to 'release-discount', Steam also often has 'Free-weekend + discount for full purchase'.
Josh has not mentioned an exact date, just that he is really close to finishing Linux support. The original idea was that the C++ version of 3.1 would be released 'a couple of weeks' after the Steam release of 3.1. Like pretty much all software/game development, delays are a recursive part of the process.