Should I contact them and tell to bring along an M-Card or would that be too confusing for them once they see my PC? Anybody with Comcast experience is the SF Bay Area?
In the past, the recommended method was to tell your cable provider that you had a Tivo. I did that and they guy showed up and said "That's not a Tivo". I just replied "It's like a Tivo that hooks up to your computer." He stated that he's never seen one before, but he plugged in the M-Card and continued through the usual process.
Since the Ceton and Prime have been on the streets for some time, perhaps it's now ok to tell them your actual device.
The Prime forums have lots of good advice on getting setup with the various cable providers.