I saw that many TreoCentral users were having SMS problems in this thread - the issue seems to be that Sprint 700wx phones are unable to send text messages to non-Sprint phones. Incoming text messages from other carriers do seem to work. I confirmed that myself today.
(Update: User timGSU notes that Sprint to Cingular should work fine and that SMS to email gateways should also work. This means that a stopgap solution is to send the SMS to a phone's "email address" instead of the actual number. We've confirmed all this here at TreoCentral.)
I called Sprint to investigate and they confirmed that the problem cropped up last week. As might be expected, they blamed it on the phone itself. That seemed odd to me, as it had always worked before. In any case, I bit the bullet and did a hard reset. No dice, still didn't work.
A call to Palm's tech support yielded similar results. They're aware of the issue and apparently working feverishly to get it fixed. It's still unclear as of now what exactly the problem is. People have been getting different answers from different representatives of both Sprint and Palm, which is often the case with a new bug - issues as diverse as Sprint network updates to 700wx firmware flaws have been cited.
The fix, apparently, is most likely to come in the form of a software update at Palm's site. It's also theoretically possible that Sprint could update some settings over the air, though both reps that I spoke with were doubtful that would be the case.
At any rate, you can head over to the original thread to complain, commisserate, and generally talk about the issue.
None of the people I spoke with had an estimated time for when the issue would be resolved. As soon as we know we'll let you know!
On a somewhat related note, some hardy users managed to extract the threaded SMS application off of the Treo 750v and load it onto a 700wx. It requires some registry edits and is not without significant bugs, so as of this time I wouldn't recommend it myself. Another reason it's not recommended: Palm has asked the original posters at xda-developers to take down the hack.
In any case, you can read about the threaded sms hack in this thread.