Of course if you turn a game console into a PVR, it needs to be able to record two (at least) shows at the same time while it is playing a demanding game. If not then it is worthless as a PVR.
My understanding is that the leaked Xbox.Next info shows that it can do that. Maybe it has dedicated hardware for that so that it doesn't slow down any gaming activity.
I also feel the Xbox should stay 1st party. Allowing OEMs to create Xboxes will just result in the overall Xbox quality decreasing.
i have a ceton card http://cetoncorp.com/
it is a PCI Express 1x card, it uses a standard Cable Card to handle up to 4 channels at a time of cable. it can share the tuner with other pc's in the home or via the xbox MCE extender you can let the pc run desktop apps or use the local MC while the xbox watches a different channel at the same time.
my system is an i7 3.6 Ghz but when i run MC or the XBMC i see almost zero load on the CPU/GPU Games put more load than the Ceton / Cable Card does.
by the way almost all of the settop boxes that you get have a cable card in the back of them.
so the hardware in the average cable box can handle 2 streams and that hardware is not high end.
( I do not know if they let you record more than 2 streams in the boxes the cable company gives)
and i can get all of the HD channels the local cable providers offer and it is exactly the same quality.