Err.. How do you suggest that someone should start? OK.. lets brainstorm..

How about writing the letter to Apple and say that you want to do an open source version of iOS for N900 and would love if they can release every bit of their source code? Call up my mate Steve Jobs and ask him for the code?

How about reverse engineering? hmmm.. hack the roms and work back?

How about... if someone ports over OpenDarwin and rewrites the Core OS, Core Services, Cocoa and Media Abstraction layers - then does a Springboard equivalent shell? Along with writing the drivers to work with the 100 odd hardware functions of the N900.

Not to mention Apple's Lawyer armies once you have gotten to a working state?

All of those months and months of effort.. to do something that you can for the total grand price of $200 USD to buy an ipod touch 4th Gen 8GB? What is it going to prove??

"I too would like to see PS3 games on my N900 too.. but no one wants to port that across.." (Sarcasm, folks..)