MOV in series with the VCC-IO pin of FT232R
If you're referring to F1, it's not an MOV (though the symbol does look like it). In USB designs, it's a ferrite bead used to filter the USB 5V line and also incidentally to have a fuse to protect the USB port from the circuit drawing too much current. Since 500mA is the max to be draw per the spec, so the fuse limits the current in case.

Unfortunately most USB ports don't have any real provisioning for limiting the current drawn. Although the USB standard requires the device to inform the host if it will need more than the standard 100mA, the reality is that many devices don't do so and just draw whatever they want(fans, lights and other little USB gizmos come to mind). So the port doesn't limit anything and runs the risk of shorting if the board you connect needs more than it can provide. Many ports can provide more than 500mA, and I've read that some apple computers provide more than 1A, but you should never count on it.

The FT232 does provide the amount of current it needs (you can change it with the EEPROM programmer), but F1 is there to ensure that the rest of the circuit doesn't do anything.