Here is an FYI:
To be able to play Mac version (even with PC disk)...
Goto
Blizzard Store
Register for an account,
Link to Registration, or click register in the top right of the above page.
After you register, click "Account" in the upper right and the click on "Games", you will receive a verification email, after you verify, you then come back and click on "Games" again.
Click "Add New Game", enter in your Starcraft CD-key. After doing so, Starcraft will be added to the games on your account. You can then come back to "Games" and click the Mac (or Windows) download link, your CD-Key is also always saved here. This download is also a NO-CD straight from Blizzard. Meaning you never need your CD again.
This can be done for all Blizzard games, I have Starcraft, Broodwar, Warcraft 3 ROC and FT, and Diablo II and Expac all on there. I can download the PC version for my desktop or the Mac version for my MBP.
Kinda long, and might be hard to follow, sorry.