What appears to happen, is that Acrobat 8 uses a separate licensing system from CS3 products, so there is a bit of a hack going on.

If you launch Acrobat 8 BEFORE entering the serial # and activating any other CS3 app, it prompts you to launch another CS3 app such as Photoshop first, then quits.

It appears that the CS3 serials numbers aren't compatible with Acrobat's standalone activation, so they worked in a bit of a hack that lets the CS3 activation do something special with it to activate it.