I just went thru this process today. I had PS 6.0 and the CS upgrade for Windows. Called Adobe and told them I wanted to switch from Windows to Mac. They verified my licenses and said I was approved for a cross-platform transfer. They submitted the case for the transfer and directed me to sales as I wanted to get the CS3 upgrade.
Sales took a look at everything and put in an order for the full retail version of CS3 for Mac and charged me the upgrade price of $199. That along with tax and priority shipping, the total came to $225.
All I need to do is fill out the Letter of Software Destruction (nasty sounding) that indicates I'm destroying the PC versions of PS and fax it to Adobe. Once they verify the form, they send me CS3.
Took about 45 minutes for the whole process.