John Nack, senior Photoshop product manger, reports the following on his blog:

"With the CS3 product cycle, Adobe instituted a '3 versions back' policy on upgrades. That is, you can upgrade to the current version of Adobe tools from any version up to three versions back. In the case of CS3 that meant Photoshop 7, CS, and CS2. With CS4 it's of course CS through CS3. The upshot is that if you want to upgrade from Photoshop 7, the window is closing, and you've got until October 15 to do so."

For all the official CS4 upgrade information, check out this page on the Adobe Web site.