My issue is similar. I have a 2-port Trendnet DVI KVM (Model TK204U) that worked just ducky on Windows XP. When I replaced a PC with a Windows 7Pro 64-Bit machine (an HP 9180f) that has an NVIDIA GeForce GTS250 with 1 GB V-RAM the whole shebang works fine until I switch back to the Win7 machine from the older Windows XP-32 machine. The display goes black, but the KB and mouse still work AFAICT.
My workaround has been to unplug the display from the KVM switch and plug it back in, that seems to work every time. uckily for me it is in a conveinant place for me right now, but I bought this particular KVM (at over $120) since it supports software switching between PC's and has DVI.
I haven't had any resolution issues, just the black screen after switching to the Win7-P-64 box.