It's been a while, but I thought RETs was a SOAP type application?
Maybe SOAP-Lite can help. I would be interested in
this project though. I had started something a while back,
but found that the lack of cooperation working with the various MLS
groups took to much time for what I needed. I also worked with a
product called Top Producer, a little better, but
the realtor was paying for the service.
In the end I did end up screen scraping from realtor.com.

Gman

Here is the developer "Getting started" Link http://rets.org/retsorg.nsf/pages/developerstart