The clamshell Zs have 640x480 screens as well as keyboards. And they run Linux. I also like the option to switch ROMs, not that I have in a while. Linux seems to be a better fit on a handheld than OSX does as well. I just think that the Z is more flexible.
I personally don't like OSX or Apple, so pardon me if I'm being negative.
By the way, this probably should have been a poll.
Edit: For those of you who couldn't tell, I'm quite sure the iPhone won't replace my Z. Also, with it being a phone, there seems to be little chance of a working Linux OS for it any time soon.