If you are hacking or doing office work (and not gaming), you can easily support an additional 1080p monitor over USB2 using DisplayLink tech. I did it in a former life using a 13" MBP and over a Belkin USB2 hub, even with no glitches.

With USB3 now, you could probably run several. I bet you could get 4 HD monitors easy.

1080p @ 32bpps is 8MB/frame. That's 7FPS max, so anything that moves a lot of pixels is totally out of the question.

Even with just Visual Studio and normal office usage, I found it very frustrating using a 1080p screen over USB. Very choppy and annoying.

"Up to two external displays."

http://www.apple.com/macbook-pro/specs/

2 external displays at the full thunderbolt display (2560 x 1600) resolution. I'm betting you could do more displays if the resolution is lower, however I don't think you can buy external displays with lower resolution that use the thunderbolt port.