Yeah, I'm guessing it is an ID either hardwired into the MiVision box (ie. aha it's the mivision box, allow 3mbit) or into it's software. If you could emulate the hardware and the software (or perhaps just the software depending on it's setup) of a MiVision box on your PC then basically the router wouldn't know any different. It would be like using a Virtual machine. The software, router, whatever, wouldn't know the difference as long as everything was emulated perfectly. While this would then mean you could basically get the MiVision package on the PC rather than the MiVision box, the next step of using the bandwidth would be the very hard part. I mean even if you worked it out then Cyta could just check their users to see if MiVision boxes were doing something they shouldn't. I mean MiVision shouldn't be downloading the latest DirectX for example, or even going outside the Cytanet domain. Would be very easy to close down (much like some ISPs do with Torrents).
It's all just talk though. While I would imagine it is possible there is no way someone will come up with it. Just not enough people. But if there were, and MiVision was at the level of Tivo or somesuch gadget, then something opensource would happen. Look at XBoxes and using them as media streamers (which you shouldn't be able to do) for example. Then again if MiVision was at Tivo level then Cyta probably wouldn't be holding us all back at 1mbit.
Cheers