The Nokia N series are not in the same device class as the Android and iPhone... the N series are very much palmtop computers and the OS is designed that way with stylus navigation etc. You don't (usually) get a stylus with an Android phone.
Well, I do think that the N900 is better than an Android or iPhone, because of its Debian-based Linux, keyboard, and "can have root". It is compatible with normal Linux/UNIX software. The new iPhone 4 and the newest Android phones do have more powerful hardware than the N900 (except if they omit a keyboard). Using OOo without a keyboard would be difficult I suppose. I don't think the device itself is in a different class, it's just running better software (supposing you want a little computer not just a phone).