Actually, I believe only AT&T blocked non-Market installs, and they are removing that restriction going forward.

Branded? AT&T is no longer locking down future devices, has enabled it on some of their older devices and it's easy to do via ADB.

re: branded

There are certain requirements to use the Android trademark. For instance, Barnes and Noble don't get to use the Android brand for the Nook.

Allowing unapproved apps isn't one of those requirements, and in practice some Android branded devices have indeed disallowed such apps.