I agree, a 12" dob is do-able if you have a truss or a collapsing version: the Lightbridge or the Skywatcher collapsible are good commercial options. My 12" is home-made and is smaller than my 4" refractor to carry around in the car...
If you aren't into ATM, but have the budget for a really nice commercial version, one of the forum members makes light-weight easily transportable dobs here in Australia. Have a look at:
http://gondwanatelescopes.com
All the best,
Dean