Using Teleflip (a free service with sign-up; Wikipedia entry), you can send SMS and MMS messages via any email client. This blog post contains AppleScript code that will parse mobile numbers in your Address Book, and add them as email addresses in a 1234567890@teleflip.com format. Here's the source (robg: with permission of the author) in case the blog post ever vanishes:
Using the script, you can send SMS and MMS messages to multiple contacts from your Mac or iPhone's Mail with ease. If you don't wish to use Teleflip, and you know the carriers for your contacts, then you can use these email address suffixes:Mac OS X Hints
http://hints.macworld.com/article.php?story=20071129131610346