Doug is right but I'd have written MARKETING IN MUCH BIGGER LETTERS. i-xxxxx is a huge success, and usablility does come into it. That said, I know people who have bought them because they are labelled as "iPhone" who will be unlikely to ever use them for anything else than making calls. Whenever people see me using my HD2, they ask me "is that an iPhone?". They have heard of iPhone, and have never heard of HTC or HD2, so I just say to them: "no, it''s better".
My sister bought an iPhone because it is a "standard" and has so many apps. Of all those she has, the only one I cannot get on my HD2 is one that tells her which carriage to get on the London tube at any station to be near the exit at the destination station. (regrettably there are now many others available only for the iPhone, even from BBC, its just that she does not have those!) My wife has tried the iPhone and says it is far more usable than my HD2.
I use my HD2 because it is more powerful and I am prepared to put up with occasional freezes so that I can cram more stuff and more varied stuff on to it than i ever could with a small screened gadget like an iPhone.

I do hate the fact that developers and companies are developing for just the iphone, and unless Microsoft get their act together, they will lose out badly. Forget the clever games and demonstration stuff, Joe Public wants funny cheap rubbish like farts and bouncing breasts and vuvuzelas. BTW, all available for your Windows Phone, but with no big fanfare - not newsworthy. Get the useful stuff like bus and tube and train timetables on there. Free maps, geotagged photos, local pubs, shops, petrol stations (with up to date prices). BBC iPlayer and Youtube and Facebook and Twitter. Let users share and swap music and photos - perhaps by touching their phones. Make it really easy. Beat apple at their own game.
iPhone or iPlayer is the equivalent to a hoover meaning a vacuum cleaner. Now it has moved to a Dyson meaning a clever vacuum cleaner, and Hoover is relegated to meaning an old fashioned basic machine. Maybe one day Windows Phone will do that to Apple iPhone.

Might I be so bold as to ask "why bother?"? I guess the only real answer is "because it's there."
The HD2 is a perfectly good phone as it is (I have one). If you have to take it apart to put WP7 on it, then you have lost use of the phone until (or if) you get WP7 working on it. Even then, there are so many things going on I doubt anyone would want a phone held together with duct tape.
Within a few months or weeks, there will be real WP7 phones to buy, and they'll have enough memory to actually be usable, unlike the HD2 which has on ly 1Gb of RAM to spread across OS and programs and data.

Interesting photo! Couldn't you find a Brit who was prepared to crumple lots of pound notes for your photo?

like the old HP hx4700, made by HTC, which had a transflective screen.

the only problem is that bluetooth appears to use quite a lot of my HD2's power to maintain.

I've had mine since they first arrived in the UK last year. Unlocked, ROM updated with HTC's releases. No issues.

Wow. I found this just before your post. it looks great and its what i have wanted since i got my HD2/

BUT I'm slightly red-green colour blind (like 7-10% of the population according to wikipedia) so the red overdues are basically not there for me. I've posted for an update so if others with the same problem can do so, we might get a today screen plugin that's usable.

What about ...
Why its bad for the HD2:
"No use for the micro SD card.
Has to work with the on board ROM/RAM only".

if its in the cloud, its expensive share. And it is VERY easy to find yourself in a place that does not get cloud access. Like for example my home office with vodafone and T-mobile and 3, or my work office with O2 and orange.

There is no way I would downgrade my HD2 for a WP7s. But I would go back to my hx4700 if it was reincarnated with a faster processor and more memory.