I took the plunge and yesterday just unboxed two of these mobile monitors. Hooking them up to both an i7 Macbook Pro as well as an older DualCore MacBook (not pro) it worked wonderfully.
Currently using the beta drivers 1.7b3 found here:
http://displaylink.org/forum/showthread.php?t=1002
The screens work great. And to answer the other question you can have two plugged into and working from the same computer.
Although I did purchase the power adapter for both in case power was a problem, from both laptops there is enough power just from a single USB plug to run the monitor just fine.
So if you are considering the Toshiba Portable monitor for a mac, I'd say go for it. My wife and I who will be taking a year long work trip love them!