The way I see it, spare batteries aren't really a very good solution to this problem. Switching them over isn't too much of a big deal, but charging both is going to be a pain in the arse because you can only charge 1 battery at a time. Yes there is a docking station that has a secondary battery port, thus getting round that problem but it doesn't fit the Desire! I read on another post that it's for an N1 and so is completely useless. Plus the whole having to power down, take the back off, swap em out and power up again is a pretty long winded when you come to think of it!
If you are going to be a real 'power user', might I suggest something like this :
USB TurboCharger 3400