Photoshop PM Jeff Tranberry addresses upgrade questions on the Photoshop team blog:
Question: Can I upgrade to CS6?
There is a special offer – good through the end of the year- which allows CS3 & CS4 owners an opportunity to upgrade to CS6.
Owners of CS2 have one last chance if they purchase CS5 from a reseller in the next 30 days (to receive a free upgrade to CS6).
Check out the rest of Jeff’s post for other useful Q&A.