Re: Probably a stupid question
(all comments posted here based on theoretical knowledge only - I have not personally rooted my current device or deployed recent versions of any OS or other software mentioned here. Caveat Lactor applies...)
Not really stupid...
Lots of ways round the lock-in problem; simplest is probably to get an Android device and then root it and install whatever alternative Android-derivative you favour. Cyanogenmod is good, apparently.
For a full-linux experience, something like this may be interesting: http://www.ubuntu.com/phone
And then there are others out there, like Mozilla with FireFox Phone, who will say they're doing something similar.
"why can't I just install whatever I like" will fall down, I suspect, due to driver issues - no smartphone (I assume) runs a desktop chipset, so you can't just deploy your current-favourite wintel build as that simply won't boot.
Options are more limited, I understand, if your hardware is an iThing; you can unlock it but there won't be the choice of alternative OS that there seems to be if your starting point is a 'droid.
See my own post below - this sort of idiocy has persuaded me to pre-order a v0.1 device from an unknown supplier; here DEFINITELY be Dragons!