Theoretically, any Mac could be cracked/hacked. If you use common sense the odds against it happening are long.
You can protect yourself by using your firewall and/or a hard wired router, downloading only from "trusted" sites, installing all security updates and being careful about what you give administrative power to.
Don't use Limewire or any other P2P service to download your software, get it from reputable sources. In addition, always keep at least your users backed up, preferably a clone of your entire system on a separate disk. And put your sensitive passwords, bank accounts, credit card numbers in a "secure note" in a new keychain or in an encrypted folder.
-mj
macjack@gmail.com