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As a heads-up, I don't believe Photoshop 6 trial (or even 7) is able to be converted to the full version, like CS/CS2/CS3 are. Also, as with the current versions of Photoshop, the serial number for a Windows version will not work on a Mac version, even if you do procure a proper PS6 disc.
Nope, CS3 is still separated out. We can hope for CS4, but Adobe tends to be a bit lame on the licensing front, so I highly, highly doubt that you'll be able to do the "one desktop, one laptop" license that Microsoft Office & Quark's XPress have adopted, and you won't be able to buy the same license to work on either a Mac or a PC.