I ran across this and glad I did...saved my .10 worth of sanity I got left:
It came from: http://kb.vmware.com/KanisaPlatform/Publishing/612/774_f.SAL_Public.html
Guest Cannot See USB Device on RHEL4-x64 Host
This problem was identified on VMware Server, but may affect other VMware products.
When you connect a USB device to a host running the RHEL4-x64 operating system, then try to access the USB from the VMware guest, the following error message appears: The Existing driver (usbhid) could not be successfully disconnected. (operation not permitted) Unload the driver manually then try again <OK>.
In this situation, you cannot unload the driver manually from the host, because it does not exist as a module—it is compiled directly into the RHEL4-x64 kernel. Normally, this situation is not a problem for VMware software, but the kernel does not support the required ioctl (control device) function to allow VMware to disconnect the device from the usbhid driver.
As a workaround, you can recompile the host's kernel with usbhid as a module, then manage its removal with rmmod. However, because the usbhid driver is necessary to allow the host and guest operating systems to interact with a USB mouse and keyboard, removing the module from the recompiled kernel may not be appropriate for your system. You may need to use an upgraded kernel or a different operating system instead.
thx
John Rose