From Adobe:
We want to make sure you're aware of an important change to Adobe's upgrade policy that may affect you.
Beginning January 1, 2013, only Creative Suite customers with licenses for CS5 or CS5.5 versions will qualify for upgrade pricing to CS6.
Due to this change, we will only continue offering CS3 and CS4 customers like you special upgrade pricing through December 31, 2012.*
* Owners of CS3 and CS4 individual products and suite editions will be eligible to receive upgrade pricing to CS6 until December 31, 2012. After that date, only customers on CS5 or CS5.5 will qualify for upgrade pricing to CS6.
You can purchase upgrades at the Adobe Store.