Ouch! Before today, I would have said they can't tell, but I spoke to a DSL support guy who as is usual asked me to use the supplied cheapo USB modem. I always assumed this was for phone support purposes, so they could train unskilled drones to do tier one support. But this agent claimed he could run extra tests from his end if the USB modem was used. Since this ISP doesn't mind you having your own network, I have no reason to doubt this claim, so I'm with Jacob.