Not long ago (see our article here), Adobe announced a restrictive policy for upgrading Photoshop CSx products. Many ones complained (of course) since that meant to buy every major release to benefit from the usual discounted price.
In a partial response to that, Adobe announced that it will offer "special introductory upgrade pricing on Creative Suite 6 to customers who own CS3 or CS4. This offer will be available from the time CS6 is released until December 31, 2012".
All details about the updated Creative Suite upgrade policy available here.
CS6 is due to be released this year.