(Rumor) has it that it is going to be Q1 (May) next year (and 28nm).
Rumor also has that it isn't a big performance jump over Kepler due to being 28nm. Videocardz.com had an article write up on this not too long ago. Interestingly enough, there are reports that NV is doing something very different in terms of memory architecture which is similar to HUMA? I don't know the details but the article made mention of this.
Anyway, Maxwell on 28nm? I don't know, man. That doesn't make sense, yet a rumor is a rumor. But it would surely make more sense for NV to put Maxwell on 20nm, although that would put the release well in 2H 2014. 20nm would give nvidia much more xtor density while being relegated to 28nm would make those gains much more limited and would not represent a big increase over GK110.
If anything, if Maxwell is indeed 28nm - I wouldn't expect anything more than a GTX 480 to GTX 580 type of jump (~15%) given similar xtor density. The real gains will be with 20nm, because that would allow NV to pack a significantly higher number of transistors onto the same die size. OTOH, Maxwell being 28nm will severely handicap NV's ability to pack more transistors which is critical for overall performance.
TL'DR: I very much agree with Shintai in that Maxwell will be 20nm and released 2H 2014.