If I'm not mistaken, N-series phones have always got decent headphones-out (circuitry, amp, etc.). Waiting for a review from an 'audiophile' who's already got an N900...
No offence, but please don't refer to the Iphone as a benchmark for sound quality.
As for now, the N900 lacks music dedicated software (like EQ and much more in this area), but that's probably the only disadvantage, which is a temporary one, since it can easily be fixed in the future. Remember, software can be updated, while hardware can't.
I'm going to embarrass myself here by saying the best pair of cans I have right now is a pair of Quiet Comfort 2s (I'm sorry, I was young! We all make mistakes when we're young, right? Right? ) and the N900 sounds absolutely gorgeous on them. It sounds even better hooked up to my Axiom M22tis.
Nice to read that. From my last N-series experience, I know that Nokia does not disappoint anyone in this topic. I would not expect something different now!