Airostep has a series of programs and services that allow Macintoshes to communicate with SMS, MMS, and other GSM- style mobile services.

Check out XMobile, which sends SMS messages to over 100 services. I haven't used it, and it appears to require the Airostep service, but it looks very nice and reviews have been good.

They also have XNet, which notifies you when your servers are down by sending you an SMS message.