It is possible
to upgrade from CS5
according to http://www.petapixel.com/2011/11/16/adobe-will-only-offer-photoshop-cs6-upgrade-price-to-cs5-owners/
Sadly it looks like it confirms what I thought I saw where they are only offering upgrades to CS5 owners. Anything below that people will have to completely re-purchase the software again and this maybe due to problems maintaining the code for older stuff.