I'm trying to find a way to mark a USB flash device in a way that I can programmaticly test for without mounting it or changing the label. 
Are there any properties I can modify about a device that will not cause it to behave/look differently to the user?
Running Ubuntu Jaunty.
You cannot modify this property, but the tuple (vendor_id, product_id, serial_number) is unique to each device, so you can use this as mark that is already there. 
You can enumerate the devices on the USB bus using lsusb or usblib.
How to mark a device in a way that can be retrived by HAL but does not require mounting or changing the label