For me its all about the web browser at the moment. There is nothing that comes close. My gripes with the N900 are more directed at a lack of support from Nokia. I know the smarty pants on here are doing amazing stuff, but at the end of the day, the 'big' changes (mms, the changes in 1.2 ect) can only really come from their end.

The Desire is stunning, in hand and having a blast on that awesome screen, its definitely one of the 3 top phones available at the moment. The N900 is literally the only phone at the moment that isn't beaten hands down by the Desire (Yup, that includes the x10m iphone, the awful, awful droid, and absolutely anything symbian based of course)

If you end up with it, you'll be ecstatic, honestly. You probably will never regret it either. Its that tight between the 2 phones. What may sway you, as I said, is the support that Android has, and to know that the software, and hardware in there, are both pretty future proof, especially with the announcement of how excellent Fro-Yo looks to be performing.