It's not a completely fair comparison as the iPhone's resolution is 2.5 times less than N900's. There is also no multitasking on the iPhone.

From what I've seen, the framerate looks good when it's not dropping frames. There seems to be some weird stuttering in the game:

http://www.youtube.com/watch?v=HKKb5EyP-S8

I think this has something to do with the OS, it's propably doing some stuff in the background which takes up CPU cycles and makes Angry Birds drop frames. Could be that the user in the youtube video is running too many apps at once or maybe a widget is refreshing itself every once in a while, causing the stuttering?

This kind of problems might get ironed out after updates to the OS, game or both.

edit: have the game myself on iPod Touch 8GB (ARM11 @ 533MHz if my memory is correct) and it's silky smooth - in comparison the N900 version is a disappointment, although the bigger real-estate is nice.