I agree with the Linux running on a MBP part... indeed, I'm planning to do so when I buy my MBP in a few months. However, I do recognize that I'm not the typical use case for a person using this device, spending most of my time in the command line, etc. For actually developing something, there's a much better ecosystem running linux.
On the other hand, I'd take iOS running on, say, the Galaxy Nexus. If I jailbreak iOS, I can easily run iSSH, install inetutils, vim, etc, and come halfway to an amazing development device with access to the whole app ecosystem. For me, that's the dealbreaker (or maker, depending on the perspective.)
I jailbroke my 3gs a couple years ago with the same idea in mind. A development platform it is not. I liked the idea of being able to connect to a server from the bus and restart a service or what have you, but for the frustration involved it's just not worth it. I'd far rather haul an Air around and tether it to my phone.
I'll disagree on this point. I've used my 3GS for two years, and it served well as a quick and dirty way to develop in... unusual places. I'm not even talking about SSHing into another computer; just to localhost, quickly using vi to change a file, then running a git commit is ridiculously convenient for someone who doesn't own an Air and doesn't want to lug his laptop around. The only problem, really, is the speed, which I agree isn't the fastest, but the iPhone 4 is really usable. When I upgrade to the 4S in a year or so, it'll probably be ready for most stuff I want to do in a pinch.
On the other hand, I'd take iOS running on, say, the Galaxy Nexus. If I jailbreak iOS, I can easily run iSSH, install inetutils, vim, etc, and come halfway to an amazing development device with access to the whole app ecosystem. For me, that's the dealbreaker (or maker, depending on the perspective.)