Why not just support Bluetooth keyboards in the 3.0 OS? This would work for all existing iPhones, and the third-party market could probably offer a variety of products with different tradeoffs for size vs. functionality.
The funny/sad thing about Macs being able to run WIndows is that it's lost on many people.
My daughter just graduated from college and had to give up her ThinkPad. At home, she needed to register for a job interview online, and we couldn't get any browser on our G5 (Safari, Firefox, or IE) to work with the site. Suspecting that this was an IE-on-Windows-only problem, I booted up XP under Parallels Desktop on my MacBook Pro, fired up IE, and we were in business.
When I turned the MBPro over to my daughter to complete here registration, she was completely unmoved by the fact that she was using XP on a Mac (under OS X): her only observation was that she missed the force-stick on the ThinkPad.
Sigh.
What I'd Like to See in the New iPhone
Is Windows On a Mac Really Drawing in New Users?