Comcast, like all(?) DOCSIS systems uses 10/8 or one of the other defined non-routable blocks for cable modems, which (if its a DOCSIS certified device) will be a bridge only and will not do NAT. If you we're NAT'ed on a cable modem system it must have been a proprietary system, of which there once was a ton before DOCSIS caught on, that Comcast hadn't phased out.
I don't believe that any of the large MSO's (and none of the small
ones
I know) are doing NAT on edge devices or the core at this point, however your point is still valid since virtually all of the ADSL lines
I can log in to the Two Wire device on my AT&T Uverse service and see the NAT configuration. It has a global IP on the Internet side and it has a DHCP server handing out RFC1918 addresses on the inside network. I can also configure some settings which allow certain applications inside to map to specific ports on the global IP. When I had Comcast, I am not positive where the NAT was being done, but my computer got an RFC1918 IP when it did DHCP.