My experience
I have a samsung focus as a work phone and an htc thunderbolt as a personal phone, I realized that I had a class 4 card in my android phone and a class 10 in my wp7 phone so I wanted to swap them since I have little to no important data on my wp7 phone, just music that I can sync again via zune and the performance of my android phone is more important to me as it's the one I play games on, watch videos etc.
I'm sure that before I upgraded to mango I was able to reformat my samsung focus sd card and I went to try it today and nothing will do it.
I've tried two different computers with different card readers
I tried a linux low dd command, I was able to find the device name and even got dd to report that it had written zero's to the card, but nothing really happened.
I never once got either laptop to recognize the card in any fashion, disk management, device manager etc, even in linux I never saw the card
I just wrote to the same /dev/... address as I saw when I put a working sd card in. I put the card back in my samsung focus and it was like I never took it out, it didn't write anything.
I've tried a number of windows tools and the simple fact is that if windows can't see it no tool will format it.
From what I can tell if your still able to see a small amount of the card 200mb or so, most likely on a hacked HD2 it's possible, but if you use a real windows 7 phone like the samsung focus and upgrade to mango it's just not possible
I'm just going to cut my losses leave the fast card in their and buy a new class 10 card for my android, you can get them for like $60 now.