Use Google Maps: locate your intended area on the map, in the "More" drop menu check the "Real estate" box, then use the ensuing checkboxes to create a pretty detailed search result, including rentals and foreclosures. What you're asking for is all there, now.
I think some aggregate, separate list of rental properties would be nice. MLS typically covers the available properties for sale fairly thoroughly but renters are left scouring through craigslist, newspapers, and shoddy realtor sites.
The strange thing about that problem is that it seems entirely an infrastructure/middle-man problem, because the people ultimately producing the data (landlords) want it as public as possible, so they can rent out their properties. There are very few landlords who, if you managed to find them and ask, would not answer yes to, "may I include your rental listing in the Big Database Anything Can Access?"
I am a Realtor in Texas and this is something I've been talking about on twitter recently. I would absolutely love to see real estate data opened up for consumption by developers.
Out of curiosity, what would you do if you had access to all national real estate data? What MLS/IDX restrictions currently prevent you from what you want to do?