I've had mostly fair response to using a USB KVM with an Ubuntu machine and XP.
There is some issue when switching back to the Ubuntu box and reinitializing the USB devices, at least the mouse.
With normal use between Windows machines, there's about a 1-2 second delay with device reinitialization. With Ubuntu, it seems to take a few more seconds to reinitialize the mouse.
I took a scant look at the issue and found someone had suggested going into a char session (CTRL ALT F1) while switched over to the XP machine, then switch back to the X session. Doing that didn't seem to alleviate the problem, as I recieved a message
drivers/usb/input/hid-core.c: can't resumbit intr, 0000:00:07-2.1.1.1/input1, status -19
I also woke up to another issue that I imagine is related, where the mouse and keyboard wired through the KVM ceased working entirelly. I've since placed a backup on the Ubuntu box if needed.
Not too worried about the issue, as I've got other things to work on/learn with this (have had Linux installed for a whopping 24 hours!). It works 99% of the time, and that's really all I'm concerned about. Of course, Your Milage May Vary.