Running iOS on the n900 is possible. But not without an awful lot of work, and inside access to the full iOS source code (I'm not sure if anyone has that!). After all, Apple ported OSX to run on Intel PCs, and the N900 has at least the same architecture as the iPhone (ARM).
That said, a few things stand in the way. For example, the fact that the N900 has a resistive screen - which means multitouch applications (which most iOS apps are) wouldn't run properly. Also, the hardware buttons - the N900 has a hardware keyboard, which it may be possible to make drivers for (there are after all external keyboards for the iPad), but no good candidate for a "home" button. Finally, apps which depend on any hardware functions (like the camera) would probably not work.
If anyone here works at Apple, has access to the iOS source code, an N900 and a lot of time on there hands, doesn't mind getting fired and probably sued, and is an expert coder, by all means do the port! With computers, nothing is impossible. But it is EXTREMELY unlikely to happen.