The black screen issue occurs in the current version of Vista (5536 pre-RC1) too. I'm assuming the mirror driver needs to be re-written to include Vista. All I see is a little 1 inch by 1 inch black square where the view window is supposed to be.
BTW, I get a similar phenomenon when using Radmin 3.0 server beta 2 as a service on Windows XP Pro x64 edition.
The viewer just gets a tiny, inert, frameless black rectangle (perhaps proportional in dimensions to the 1920x1200 screen it's supposed to be viewing) in the center of the screen on the viewing computer (running regular XP Pro SP2).
I hope this is fixed soon--Radmin 2.2 doesn't work for me since I use Office 2007. The menus (command button induced, right-click induced, or fade-in) don't show up on the viewing machine. Very problematic--hard to use without those menus.
For some reason I was thinking the server was the problem (the server, with its XP x64 edition OS, seems more odd than the viewer), but of course you're right, it could just as well be a viewer glitch, so far as I can tell.
I sure hope they fix it soon, though. I really, really want--and expect (eventually, and sooner rather than later, given that these are ever more utterly mainstream products)--a version that works will with XP Pro x64 and with Office 2007.