All the receivers are capable of HD. It's just the main receiver (VIP1216) that is fed via cat5 from the 2Wire gateway. All other receivers (VIP1200, up to 5 in total) are fed via coax from the 2Wire using a technology called HomePNA. The 1200s are capable of being fed with cat5, it just ties up more ethernet ports on the 2wire (and like it was said, apparently cat 5 costs way more)
The installer is supposed to run a coax line test to verify that the coax connections are good. I think it checks bandwidth, signal loss, etc. If it fails, it might be due to a bad F-connector, coax wire, barrel, splitter, or any combination of those. Makes things very frustrating if you can't pinpoint the cause. Can take up a lot of time!
, MTS standards are that the HPNA test *must* pass before the installation can be considered complete. They even want the results emailed to them for every customer (well, the one's who have more than one TV.) I think it's to increase reliability/reduce service calls.
You should see what the test screen looks like. Especially for someone with 6 set tops. There's two way communication between each receiver, so the test program shows arrows going every which way. One red line and the whole test fails.