I think this should be high on the Google search results for "HTC Desire battery life" searches.
I've been playing around with my Desire and battery life for a couple of months. I've found a massive battery hog... Hotmail.
If you are receiving Hotmail with Exchange / Activesync, whatever e-mail program you use will a ton of battery. It didn't matter if I had it set to only sync e-mail, only 3 days worth and to only do so once per hour. Also, I probably only get one e-mail per day from Hotmail, which I normally just delete (junk mail).
My battery life is normally 2 to 3 days with the phone, but less than one day with Hotmail set for Exchange / Activesync.
Fix: use POP for Hotmail. Then, no more battery problems. It will change from using that e-mail program for 2 to 3 hours per day to 2 to 3 seconds per day.
Also, I have tried 4 different mail apps. The HTC mail app, the original Android mail app, Incredimail and one other one.
SystemPanel Lite would say that the mail program using Hotmail with Exchange / Activesync would use an average of 1.5 to 3 hours of FOREGROUND cpu usage each day!