In the past, Adobe has had a very generous upgrade policy. Users could usually upgrade from a version of the Creative Suite that was 2, 3 or even 4 versions back, and still qualify for upgrade pricing to the latest version.
But in November, Adobe announced that this will change. Starting with CS6, customers will need to be own the previous version in order to qualify for upgrade pricing on the latest version. Adobe received some negative feedback about this policy from the community, and apparently took this feedback to heart.
Adobe announced today a Special Upgrade Offer that will allow users of CS3 and CS4 to upgrade to CS6 at the upgrade price "from the time it is released until December 31, 2012." Adobe is saying that CS6 is coming "in the first half of 2012."
Note also that in addition to offering CS6 as a normal "perpetual desktop license" Adobe will also be offering monthly subscriptions to Creative Cloud, which will include access to all the software in the Creative Suite and more.