From - http://photoshopnews.com/2006/12/14/photoshop-cs3-beta-qa/

Q. If I buy Photoshop CS2 today in order to get access to the Photoshop CS3 beta, will I get a free upgrade to Photoshop CS3?
A. No. The rich feature set and productivity enhancements of Photoshop CS2 already provide a strong upgrade value, and the opportunity to preview the upcoming CS3 release is an additional bonus. In addition, customers who are still using Photoshop version 6.0 or earlier will benefit from taking advantage of a more liberal upgrade policy for Photoshop CS2. Adobe will not offer upgrade pricing more than three versions back on Photoshop CS3. Go to www.adobe.com/products/photoshop/index. html for more information on Adobe Photoshop CS2.