You're kidding, right? The difference between PDTV and 720p is night and day!
Agreed. My parents have a 37" HDTV (1080i IIRC). I compared Planet Earth in SD and HD by plugging in my MacBook Pro straight into the TV. The difference was incredible (and it should be noted that the SD version of Planet Earth I had was better quality off the computer than what we originally watched from our set top box too!). Just look at the leaves on the trees between the two and the difference is impossible to miss. That's before you go on to look at anything else.
However we do not have a Sky HD box as - as has already been said above - they're too expensive at the moment if you're just after the HD channels. As I don't live at my parents anymore and they don't watch much TV (mostly movies) they're not really interested in the Sky package, although they did debate it for the HD channels (well, the BBC one). They decided that there is currently nothing of that much interest to them on the HD channel to make it worthwhile getting the box & satellite yet (Planet Earth was about all they were interested in).
And therein lies the problem. The expenditure required to switch a program like Top Gear (think about how many cameras they have - a few per review (onboard shots + outside shots) plus several reviews being done at one time possibly) is quite large, and given that not many people have made the switch to HD yet it doesn't make sense.
I think it's a great shame. Having experienced it myself I can't wait for the proper switch to HD. Unfortunately I cannot personally afford a HD TV right now myself and this is a problem a lot of people here have... But when there's more quality programmes in HD I reckon the amount of people using it will skyrocket... (I will be one of them - somehow!)
Really it does need flagship programs like Top Gear to be early adopters of the technology to help push the technology forward. But as it's paid for by the BBC, they do have to be careful with their budgets. Like others here, and I believe the TG crew share this feeling too, I'd rather see the money spent on the reviews and stunts etc at the moment, but I really do look forward to the day when we get TG in HD. I just don't think it's a priority at the moment as it's not a show that will benefit a great deal from HD - unlike Planet Earth...
Sorry, a bit of a convoluted post and I can't spend time sorting it out now!
Basically I'd love to see TG in HD but just don't see it happening in the near future...