Only Intel knows for sure, but here are some interesting links:
This July 2009 article on HotHardware shows a plan for launching a "Postville-Refresh" in Q2 2010. Clearly that doesn't appear to be happening.
This January 2010 article on Anandtech points to a new SSD based on 25-nm Nand in Q4 2010:
Doing some basic research, you can find that:
Based on the 1 year, 2 months it took on 34nm, that would be April 2011. Assuming a one-quarter pull-in (20% improvement), that would put a new SSD based on 25-nm in Jan 2011.
Anyways, those are just guesses, but you can bet you'll hear a lot of noise as the G3 release gets closer.