Where can I find an online service that will allow me to subscribe to
XML data from real estate listings?  I would like to get the listing
information for all of the United States based on city/zip code in XML
form to integrate in my application.  I need all of MLS.  Have already
checked OpenMLS but that is not what I am looking for.  For example,
check the Yahoo! Real Estate section and do a search for any region. 
HOW does Yahoo! get the real estate listing so that they can put it on
their site? The XML interface I am looking for must provide enough
results and quality of results for searches such as in Yahoo! or
Realtor.com.