You could hack Maemo to install a different GUI. It would take a lot of work but you could make an App that switches to that GUI and exits it back to the original GUI when you hit the App again in your GUI. (Look at Openpandora for help). In the new GUI, you can do anything you like with the UI depending on the GUI you have selected. Something like the iTouch shouldn't take much resources (from the CPU) or effort to construct (I think).

So it is definetly possible and perhaps easy if you are a good coder, good at graphical presentation and have plenty of time on your hands, but chances you aren't. If you know your stuff, choose a good and supported GUI (I have no idea but heard great things about Lxde) and make the App to switch into it and out safely; someone/group with the rest of the skills could complete the project.

This would mean your N900 will have an iTouch skin: beautiful and simple UI but none of its Apps would actually work on your phone.

I think if the idea/project came to life it would be very beneficial since it is the UI that causes people to turn away from the N900 and starts flame wars and feeds the trolls. Your project could evolve into something neat (and you would get the glory) for instance proper portrait and landscape functions?

Edit: And you could have the App running from internal storage (save power & performance). It would be something that doesn't change your core files just gives you a different graphical presentation of your phone (look at iPhone Themes for WinMo6). The App would just rearrange your executable files into a 4x4 tile and pages, so it would be quite light. It would auto-save the order of the tiles and pages into a file in itself each time you closed it. When opened it would restore to that last tile arrangement state. You could arrange the into your desired sequence by holding down anywhere on the tilescreen (homescreen?) for 3 seconds and it would cause them to jiggle and be able to arranged in any order. You should be able to add folders, individual contact profiles, links to a website (the icon would be user made/used browser) by clicking the plus and browsing for that file. You could even leave out gaps! Holding down for 3 seconds again would stop the jiggle. You could flip pages (with kinetic scrolling) by scrolling. All of this could be achieved with the resistive touch screen. Including pinch-to-zoom could be implemented for apps (like in WinMo) by placing two fingers on the screen and move one of it in/out wards for the zoom.

I hope that clarifies/details the idea!
PS dont need iPhone Apps, you can have any* Maemo/MeeGo/Ubuntu Apps!