Difference Tbred A and Tbred B?

Is there a difference between them?
Are they made for all speeds?
I know that the B variant is better in overclocking.
Is there a Tbred A & B proc. of the XP 1800+?

There will be in the future. the Bs have an additional layer of metal and about 400k additional transistors, which allows the chip scale (reach) higher and run cooler and use lower vcore and still remain stable.
There will be XP1800+, 2000+ 2200+ as Tbred Bs and As, but the 1900 and 2100 will only be As...

One more question then. If a Tbred A works on a mobo, will a Tbred B also work then?
And vice versa?

Yea, I think there should be no reason for one chip to work and the other not to, because they still have same externel design.