Nathan over at InsideGoogle mentions a post by Devin Reams who points to a portion of the Google Firefox Send to Phone FAQ: FAQ that says, "we [Google] might also log the text of the message you send, in order to investigate and correct technical problems with the service."
The wording in the privacy section of Google Toolbar 4 documentation reads,
If you send text through SMS using Send To feature of the Google Toolbar, Google logs the number and carrier the message is sent to, and in some cases may record the text sent for debugging purposes.
I don't have time at the moment to check, but I'll try to find out if other services like Vazu, Yahoo's Text-to-SMS service and some of the other web-to-SMS services have similar policies.
Postscript:
You can check for yourself. Here's the privacy policy from Vazu and Yahoo (mobile devices).
