Can I play Warcraft 3 (DotA) on a Mac?
You can create an account at www.battle.net and then tie your Warcraft 3 CD to your account.
Once you have done this, you are free to download the client on Windows or Mac at any time.
There's no need to buy different copies :)
You can download it directly from the app store for 9.99$. Have it in my computer and work perfectly.
Warcraft 3 cannot be installed in Mac OS X 10.7+ they no longer support the emulation framework necessary to run PowerPC applications.
More information can be found here.
UPDATE:
Warcraft 3 has been ported to work with Intel-based processors and can again be installed on modern Macs.