Anyone know the difference besides a $100 on Amazon?
Yes, the /B is the latest "Santa Rosa" revision of the MacBook.
So while the processor is the same speed (2.0Ghz) the B revision will have a slight performace edge over the A. The most notable difference however is improved graphics which make the MacBook go from a "no-buy" to an "acceptable" for light gaming. Intense gamers will still be disappointed either way.
Secondary, the maximum memory is 3.3GB on the /A and 4GB on the /B.
There is still a $75 rebate on the /B since it's Amazon selling it. Furthermore the base MacBook shouldn't be more than what apple sells it for, namely $1099, or $1049 today on black friday.
Don't consider buying the base macbook,
last revision, for $50
more than the new one is from Apple, or
$125 more than the NEW ONE from Amazon directly (including the rebate, which is no-hassle).
Long story short. Get the /B for less money.