Foiled once again, but I think I found an answer to your problem Dave. Check it.
crushing_belial,
You will find that this issue occurs when you allow the program through the OSX firewall (odd sounding, we know).
Putting the program allowance into the firewall modifies the executable for the game.
Uninstall Starcraft, go to your OSX firewall and remove the program from the allowed list and then reinstall. This should allow you to play.
crushing_belial,
The firewall hasn't been reported to cause this issue on Tiger, but what John stated above is true for Leopard users.
It should also be stated that the OS X Parental Controls can cause this issue as well on Leopard. Much like the firewall, specifically adding applications to Parental Controls will modify the application file itself.
Since you are on Tiger and multiple reinstalls has failed to resolve it, usually corrupted RAM or hard drive will be the next likely cause. I've never seen a Trojan/virus cause this issue on a Mac.
I suggest running a Disk Repair and the Apple Hardware Test. Insert the install CD that came with your Mac and restart the system. While it boots up, hold D to enter the Apple Hardware Test. Run this in Extended mode, please.
Using the same CD, you can run a Disk Repair through the Disk Utility which is similar to the utility in OS X, only a bit more thorough.