Found a fantastic documentation for the Google Voice API to send SMS messages, Google does not have a documentation of how to use Google Voice, in fact is always hard to find any good documentation to any of their products besides Google Wave, any we all know where that project went.

This will give you the rest api to place a call as well as send an SMS, there is possible as well to read your SMS messages. We should all thank Chad Smith.

http://posttopic.com/topic/google-voice-add-on-development

For SMS

POST /accounts/ClientLogin accountType=GOOGLE&Email=[google account]&Passwd=[google password]&service=grandcentral&source=[your app name]

Sending an SMS:

POST /voice/sms/send/ id=&phoneNumber=[number to text]&text=[URL Encoded message]&_rnr_se=[pull from page]