You can save up to 27%* on Adobe CS5 by purchasing the relevent upgradable CS4 product - and then upgrading for FREE! It's that simple.
If you purchase/have purchased a CS4 product anytime between 12/04/2010 and 11/07/2010 this means that you have aquired CS4 in the "post-announce period". Therefore you are eligible to upgrade to the relevent CS5 suite or product, thus saving you up to 27% on the price.
You can buy from any Adobe reseller.
See also Amazon's page detailing the free upgrade offer (see comment 2)