I'm not sure about what they exactly mean by unlocked G1.

The G1 I have is SIM-unlocked, but clearly the firmware has got T-Mo-specific apps on it. I'm guessing, given sufficient clarity of how 'unlocked' the Dev version is, but based on what I have read so far, this simply means that the Dev version

a. supports unsigned firmware,
b. comes with a generic version of Android (i.e. no T-Mo specific apps, e.g. MyFavs, WLM/YM/AIM IM services - similar to the UK version then?), and
c. saves you the hassle of having to get it unlocked on your own (and not having to call up T-Mo for unlocked codes, thereby reducing their call centre capacity for people with genuine phone/network problems)

That said, I do wonder if it would be possible for people to extract the firmware from the Dev version and make it available to people around the world who have unlocked T-Mo version who want to update to a generic (not jailbroken) version