Many devices are being announced with 1.5 but will be upgraded before or soon after launch, as I see it many rooted phones that came with 1.5 or lower are almost all at 1.6. Some $100 devices announced will never go higher than 1.6 because of performance issues unless Google actually optimizes newer versions of Android.
The htc Magic(Mytouch) and htc Dream(G1) here in canada only officially have a 1.5 update and no 1.6. But in the USA they now have 1.6. I have a Dream and I rooted it and have firmware 1.6 but I have run 2.0, 2.01, 2.1 on it but performance is not too great on it. With 1.6 I can play apparently slow games on it at a decent speed.
Google also stated that they will update most android devices to have 2.1. So I would think that All Htc and Motorola phones will end up with 2.1. Just give me a moment to find the source of this.
All US Android phones to have 2.1
Devices that have a custom ui are: (These are the devices that I know of but there are more)
Sence UI
HTC Magic in canada
HTC Hero
MotoBlur
Backflip
Click
Click xt
There are custom roms that enable sence ui and motoblur for motorola phones and htc phones. For example I have run sense ui and motoblur on my Dream.