At a guess, the majority of N900's would be those looking for an iphone alternative but with keyboard an no at&t.

With a much smaller percentage buying because its 'open source'

I feel the droid may steal quite many potential customers away from the n900 when they see:
- its slimmer
- larger screen
-xenon flash. Real camera replacement for low-light shots
-slightly lighter?
-is 'somehow' affiliated with google. Ovi Maps doesn't even come close to google maps in USA.
-developers can use Android NDK to write c/C++ apps now, so casual developers have an excuse to buy it too.