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.
Upcoming Webinar: PPC Pause and Reflections for 2013
Thursday, December 12 - 2013 was a major turning point in search advertising. With Google's Enhanced Campaigns and Bing's innovative Smart Search capabilities in Windows 8.1, now is a great time to pause, reflect, and plan for the new year. Webinar attendance is free. Sign up today!