There are, however, external videocards such as the Matrox dualhead2go which will allow you to drive dual displays from a single displayport... Not cheap though
to the best of my knowledge the matrix adapter is restricted to a resolution lower then the thundebolt can achieve and because of the way the resolutions are set up on a thunderbolt you will end up with something much lower.
I havent seen any usb / external video card adapters that can put out apples thunderbolt resolution. I know of 1 card thats close, but again because of the way the ATD steps the different resolutions you end up stepping down to something much lower.