People seem to be confused about what to do, so here is a quick run through.
The correct procedure is to do the following:
1. Download the software
2. Go to the adobe site (or here if the previous does not work for you) and log in or setup an Adobe account if you don’t already have one
3. Enter your CS2 serial number. This can be for the whole Suite or just PS. If you don’t have this, say so
4. You will get a CS3 serial number. This will be 30 day limited if you don’t have a CS2 number, or until the CS3 ships if you do
5. Install CS3 Beta and give it the CS3 serial number.
That’s all there is to it.