I think this is best pre order deal on the HTC desire out there at the moment. It works out as £540 across the whole 18 month period with 300 mins, unlimited texts and unlimited mobile web.
Deal is with T-Mobile and is as follows:
£25 Unlimited Text + Internet Boosters on T Mobile
18 month contract, 300 Minutes, Unlimited Text + Unlimited mobile web
£30.00 per month Line rental, 3 months Free insurance
Works out at £540 over 18 months for same service as others posted over 24 months, no upfront cost for the handset and you're out of contract 6 months earlier.
This handset is supposed to be the same as the Nexus one but doesn't look like it's available until 1 April 2010.
As it's T-Mobile, you should also be able to port your number.
Poss £30 from TCB as well.