Ok, I've been having problems too, but I think I've sorted my one out. I'll first go through the symptoms (apart from the obivious that the battery life is poor).
1. Go to Menu>>settings>>about phone>>battery. If your "awake time"="up time" then your phone isn't sleeping.
2. Type *#*#4636#*#* into your phone dialler. This will bring up the testing menu. Go to Battery History, then press "other usage" and select "partial wake usage". This shows what is causing your Desire to remain awake (even though the screen is off). More likely than not, this will show "calendar" as the biggest use. If this is the case go to the next step.
3. Go to Menu>>Settings>>Accounts & Sync. Now select the flickr account and then "remove account". Hopefully this should sort it. I don't use flickr and only logged in once hence why it's here.
Hopefully that should sort out 90% of you. If you have the "awake time"="up time" problem then at least you know that the problem is your Desire not going to sleep. And then the 2nd step will tell you what the problem is.