Got the BlackBerry VS9 Plugin running in Visual Studio 2008 running in Windows 7 x64.
Not sure what of these steps are really necessary, but this is what I did:

Copied "Research In Motion" directory from "Program Files (x86)" to "Program Files". This gave me a different error.

Discovered using SysInternal's ProcMon that it was trying to launch RIM.Net.Tools.ScriptHost.exe from the root, so I copied that and all the other stuff in the plugin's bin folder to the root, yes, C:\.

That gave me another error, so then I set VS2008 to run in XP SP3 compatibility mode.

Tada!