More an extension of current PL II technology than something totally new, IIx will upsample up to 7 independent channels from stereo sources and generate up to 2 additional surround channels (yeah, stereo rears) from discrete 5.1 material. Hopefully, like any Dolby licensed technology, it will eventually become ubiquitous, appearing in even the lowest priced 7.1 gear (as PL II does now).

Pro Logic IIx press release:

http://www.dolby.com/events/CEDIA/PLIIx/

Pro Logic IIx FAQ (very thorough):

http://www.dolby.com/events/CEDIA/PLIIx/faq.html

Best,
Sanjay