It all depends on home many messages you're sending. If you're sending in the tens of thousands per month, there are companies that'll pay you to send them for you (and include an ad)
If you're just sending a few, you should send them using an email. The only downside of this method is that you need to know which provider they use, which means either asking the user, or looking it from an expensive DB.
If you do know which provider they have, you can look up the email here
http://www.livejournal.com/tools/textmessage.bml?mode=detail...
If you don't want to ask, you could use Teleflip, who will do the lookup for you. Just email phonenum@teleflip.com
There was an earlier discussion about this, but we don't have a FSQ [1] or search feature, so I don't blame you for missing it. Take a look at http://news.ycombinator.com/item?id=68227
Let us know which way you end up going!
[1] http://news.ycombinator.com/item?id=97491