Adobe’s upgrade system has historically allowed you to upgrade from the last 3 versions. This means if you have CS, CS2 or CS3 you can currently upgrade to CS4.
Today Adobe announced that CS5 is coming on the 12th of April. Once that is shipping and assuming Adobe maintain the same policy on upgrades then you will need CS2, CS3 or CS4 to upgrade to CS5. If you have CS it will most likely mean a whole new purchase, with no upgrade option.