I have done this and it was pretty painless. One hint I picked up somewhere when researching the issue: When you call, call Sales, not Support. Sales seems to know how to do this, some people report that Support doesn't. The lady I talked to knew exactly what I wanted and was very efficient.
You fill out a letter agreeing to destroy the Windows version and deinstall it, fax or email it back, and they ship you the Mac version in a retail box.
Cost, as others have remarked, depends on whether you are upgrading to CS3 or already have CS3 for windows. If you already have CS3, cost is nominal (S&H, essentially...I think it was < $20).
I believe the last issue with CS3 under Leopard was fixed with the recent release of Acrobat's upgrade (8.1.2 IIRC). I've not had any problems with it.
The letter is worded so that, as commented above, they expect you to uninstall the s/w before getting the new.