Since my mother got her iphone 4 I have been a bit amazed how well it actually works. Everything works flawless and the application support for it is super-big. BUT, as AgogData says it's completely dependent on iTunes which is (at least on windows) a resource-hogging mammoth.
The thing that I envy most is the official client for spotify, especially the offline-support. Qspot for the N900 is kickass in many ways, but the offline-feature is something I really miss.
My opinion is that the N900 and iPhone rocks in different ways!