The N900 is not directly competing with the iPhone. The iPhone is meant to make calls with and post to twitter, very little else. Comparing the two is like comparing a Netbook with a Scientific Calculator. The range of applications being made available for Maemo4 (the N900’s operating system) are MUCH wider than that of the iPhone.
I own a 770 tablet and have trialled an N810 and honestly, I’ve yet to see a killer app from Maemo.
But I think that will quickly change with the added utility of a GSM radio in the N900.