I just bought this:
iGadgitz Black Neoprene Sports Armband for Apple iPhone: Amazon.co.uk: Electronics & Photo
it's for the iphone, but the dimensions of the 2 handsets are quite similar so I thought I'd give it a go..
It arrived this morning and I'm very pleased with it.
The Desire is a whole 3.5mm taller than the iPhone, making this a bit of a tight fit, but that actually counts in it's favour. Because it's more or less "jammed" in, it feels really secure. and the hole to put your phone inside is covered by a large velcro hatch on the inside of the band (against your arm when worn) so there's ZERO chance of it falling out.
I installed cartiotrainer yesterday and today I'll be giving them both a good test on a short run near my house.
I recommend this armband for the HTC Desire.