Need a PCI USB card that works in gadget mode

something with one of these in it:
http://www.plxtech.com/products/usbcontrollers/net2282

Or something else supported by linux upstream : http://www.linux-usb.org/gadget/

has anyone come across anything?

Isn't that chip used when someone wants to take a previous PCI (or Cardbus) design and implement it as an external USB device?

Does it have to be x86 Linux? A lot of the Beagleboard-type things have USB slave.

Peter

High throughput, or very low-speed requirements?

For low-speed, a simple Arduino-like board could work.
Eg."Sparkfun Pro Micro" provides USB target with a HID profile, which can then be interfaced to from the PC over a serial link very easily.

But I'm guessing you want something with a bit more oomph!