From the L3 MMW website:
http://www.sds.l-3com.com/products/mmwave.htm
"Privacy Protection: ProVision offers multiple levels of privacy protection that can be customized to support customer-specific operational procedures."
It's not so much that I trust the TSA. I do trust L3 though to deliver scanners with saving disabled, provided that actually was one of the TSA's requirements. And L3 is perfectly up front about the fact that they can be configured for various levels of privacy.
As incompetent as the TSA is, how often have they flat-out lied about something? Knowing government agencies, if they DID save images and TSA didn't want us to know about it, they'd just tell us that the functionality of the machines is classified.