me too!
i've got a 2011 15" mbp. when using the kanex miniDP to hdmi adapter (which i bought for my 2008 mbp... it has three inputs, miniDP, optical audio in, and usb for power, one output hdmi) it works perfectly. when using a single miniDP to hdmi adapter that i bought from macsales as well as another moshi brand i got yesterday, neither of them work. i need the limited usb ports...
since the older kanex works, i'm inclined to think apple has changed something. i have NOT tested either of the miniDP adapters on a 2010 mbp. i don't have access to one.
when i plug in the macsales adapter, the screen goes blue for a second on the mbp. then nothing. the moshi does even less... nothing.