In the US we can put our SIM card into any phone for some networks like AT&T, cingular, T-mobile, but some networks use a custom system, like Sprint for example.

Some phones you get are locked, but most carriers that I am aware of will unlock it if you ask them to, and locking is some what of an theft protection.

I have an unlocked iphone (I use it with prepaid t-mobile, I pay $25 a year) but it sucks, especially because it's a longer process to upgrade to the latest firmware, and there is always risk of bricking it. Most of the people that I know that were modifying their iphone for the extra apps ended up restoring it and not bothering to do it.

There was a point when I was interested in developing for it but it was such a huge hassle to setup a development environment outside of having a mac that I gave up.

The iphone really isn't for me because of the way apple is, and it also isn't for my parents or my brother, none of them have shown any interest, and all of them are fine with their carriers. Only people that I know who are more tech savvy have already bought an iphone, but even some of them are not interested, and some of them are even a little pissed because they paid more for their phones and now have to deal with an old iphone that has to be sold really cheap.