Anyone know the difference between...
 

Dura Ace A-type versus B-type, specifically where chainrings are concerned?

I believe one is designed for use with 42t inner rings, while the other is designed for 39t inner rings.

http://www.sheldonbrown.com/dura-ace.html#chainrings

• Set "A" uses a 42 tooth small ring, and a corresponding "A" type 53.
• Set "B" uses a 39 tooth small ring, and a choice of "B" type 52 or "B" type 53

yea shift pins and gates are in different spots, depending on the inner chain ring.

I have mixed A Type big ring with B type small ring with no discernible negative consequence.

I agree with all the posters above. The A and B designations refer to the chainring combinations that will provide optimal performance. But it's not like your bike will explode if you mix an "A" big ring with a "B" little ring.