My two cents on the fluidity of the iPhone UI:

1. Prioritization of tasks
- if audio shutters, it is quite annoying
- if you receive a phone call, waiting seconds is not an option to see who is calling or to put the ring on mute
I would imagine these are not handled in Maemo 5.

2. Prioritization of UI fluidity
- you can make certain tradeoffs and those were made in the iPhone
(I'm not talking about multitasking, more what ARJWright wrote)

3. Setting objectives and working towards that without compromise
- Nokia understands many things about mobile and the future. Unfortunately, Nokia does not have the execution capabilities for objective-based working. They release a 100 good ideas a year. Very few of those are actually seen to completion. Nokia prides itself to have a discussion-based culture, a company of many voices. There is no leader.

4. Quality Assurance process
- Nokia has no proper Quality Assurance. I'm not talking about missing functions, I'm talking about not working ones. Each and every one of Nokia's releases is full of bugs (be it Symbian, Maemo or web-services). Partially it is due to having no leader, but there is also a fundamental flaw somewhere in the process. I remember listening to closed-beta testers (bloggers) of some services, like Nokia viNe. Bugs, which were listed 3 months before release were never corrected.

5. Learning curve & consistency
While the posts here about transitions were very good, we miss one additional time factor: the time the user needs to position for the button/UI element. It is dependent on 2 things: unconcious moves and concious ones. If you have elements always at the same place (e.g. top left and right corners of Maemo 5 UI), it becomes second nature to close or submit things. If you group (and force) buttons/UI elements to look the same, it gives a shorter learning curve (confidence) to the user, therefore they will not spend half a minute figuring out what to press. Inconsistency destroys the confidence, so in user's mind all things need to be double-checked.

I believe the whole fluidity thing has nothing to do w screen resolution or GPU or anything like it.