At work, we compared a desire and my n900 for a few hours in different things. My N900 runs at 800mhz, with official software. The desire with the latest update...

Camera - Desire, sharper, better colours
Video - N900. Better framerate, more detail.
Media - N900. Plays almost anything you throw at it. Speakers are great. Not a full reliable app for Desire for divx ect. Yet.
Touch screen - Draw. As usual, resistive is more accurate, capacitive more sensitive and fluid. Only just though.
Internet - N900, easily. Flash is miles ahead too, the Desire runs out of memory constantly, and the lack of stylus is a pain on a high res screen.
Memory - N900. Obviously.
Looks - Desire. Slimmer, nice finish on the back
Display - Desire. AMOLED looks beautiful. High contrast, vivid colours. Not much between, but enough.
Apps - Desire, as in more variety, better system to deliver the apps, and you get the feeling it will always grow.
OS - Draw. I love android for its usability, but you cant beat how open maemo is.
Social networking - Draw. I dont give a rats about facebook, but they both do well, both have widgets, the n900 has the bonus that is can handle 'full' facebook with all its flashy glory. Twitter ect is also so very lame, but is equally supported on both.
Call quality - Desire. Excellent speaker volume. Clearer then n900
Games - Desire. Better choice of quality games. Although Duke certainly puts up a good fight.
Battery - Desire, although still not great.

N900 gets super brownie points for an awesome qwerty.

By the time I was done, the Desire really didn't convince me in anyway that it was a 'better' all around phone. Of course, everyones looking for something different, but if you already have an N900, you already have the best out there.