From the Firefox Send to Phone FAQ:
When you send a message using Google Send to Phone, Google will receive and log the IP address [?] and one or more cookies from the computer you are using, as well as information about the mobile phone you are sending to, including the phone’s telephone number and carrier. In addition, we might also log the text of the message you send, in order to investigate and correct technical problems with the service.
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 can understand logging the carrier number and a few details of random text messages for debugging purposes, but I can’t imagine what is accomplished by logging whole text messages. Seems like Google is just covering all its bases in case it wants to collect more of its precious “information”.