I can confirm that this is the case as I currently have the setup now -- HDMI and MiniDisplay port, both adapted to DVI into 2 Dell UltraSharp U2711 27" monitors. The desktop is successfully spanned across both monitors, even though they are hung in an up/down configuration (using the monitor controls to flip one display I believe).
What I don't think anyone here knows is (but am hoping they do
): Is there a performance bonus you can get by instead a 3rd party device to use just one of the display ports on the back of the Mac Mini (either HDMI or MiniDisplayPort) instead of using both HDMI and MiniDisplayport at once? I've noticed a little sluggishness with the Mini in this new configuration and I want to see if I can put the burden of that on an external device.