I can't wait until we can simply pick and install our operating system of choice after we purchase a smartphone. After all, we have that flexibility with PCs. You can install Windows XP, Vista, Windows 7, Ubutnu, or even Mac OS X (if you're creative). Why can't we do that with our smartphones?
Actually, we can to some extent. I know that many people have been installing Android ROMs on HTC smartphones like the Touch and the Touch Pro2. I don't think I've heard anyone installing the iPhone OS on any other device.
I've played around with custom ROMs in the past, but this can be a time-consuming hobby. Each time you install a new ROM, you have to reinstall all your apps. New ROms are constantly being generated as users within the community identify bugs and fixes. At some point, maybe I'll install Android on my HTC Touch Pro2 (which is currently running Windows Mobile 6.5). Then again, it would probably be much easier to simply go out and get a Motorola Droid.