USA houses  

USA! USA! is a big place.
Some ideas:
Fallingwater / Kentuck Knob
Wharton Esherick house
George Nakashima Woodworker
Philip Johson Glass House
New Caanan, Connecticut
I'm sure there's tons in California.
http://www.preservationnation.org/travel-and-sites/sites/northeast-regio...

check out retrorenovation.com...
check out retrorenovation.com. Yesterday they just started a list of places like this over the whole country to go see.
retrorenovation.com is a blog and does not sell anything so this isnt an ad.

Cape Cod
Another possibility...
http://www.ccmht.org/

Houses
If you are anywhere near Grand Rapids Michigan You owe it to yourself to check out the Meyer May house. It is a Frank Lloyd Wright design and it is a mature prairie style like the Robie house. Except it has been painstakingly restored to immaculate condition complete with all of the furnishings and floor coverings. The Steelcase office furniture company owns it and spent God only knows how much restoring it It is beautiful and the best part is you can tour it absolutely free.

FLW
The Auldbrass plantation in South Carolina is open this fall also. They open it once every two years to the public and that's it. You might want to check that out if you can.

Grand rapids http://www.savewright.org/index.php?page=28
Tours of homes at this years fllw conference.
How are you getting around?

Thanks all. I'm currently in...
Thanks all. I'm currently in Chicago, have been in NYC Philly and DC.
The excellent architecture foundation in Chicago, whilst with a strong concentration on offices and skyscrapers, also takes you to farnsworth house and some FLW places, so I'll hit that up
I'm getting around by coach and plane and potentially Amtrak so I fear that many places will be out of reach, unless I can take a cab for a reasonable amount or go on a coach? (unless someone wants to go on an excursion and has a car lol)
Chicago is a good hub for the rest of America, so just figuring out where to go and what to see in the way of modernist residential architecture

Yes
A lot of these famous modern homes are out of reach from mass transit.

If you feel like spending $300 for it...
... the Eames House will be open for interior tours on the afternoon of 22 June.
If you don't want to spend the money, you can see the exterior for $10 any day. There's a bus stop just up the street:
http://www.eamesfoundation.org/how-to-visit/

dwell
You might like to check out the latest issue of Dwell, which features modern design in America.

Don't forget the Lloyd Fadem...
Don't forget the Lloyd Fadem home in Tulsa Okla.
http://www.okmag.com/May-2012/Historical-Hunter/

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Thanks Mark
It has been fun...
Thanks Mark
It has been fun collecting for over 30 years and it was fun to find the perfect house, someone ask me what I am collecting now and I told them just money.. I wish!!!!
I can't stop!!!!
I am collecting mid century travel posters, mostly worlds fair from 1964 from the big 3 airlines plus New York travel posters, so watch out as I am ebay crazy and can't stop buying them. truly a design addict here.