I understand that a Rev change points to hardware changes but D-Link swears up and down this did not happen. So, I'm just giving them the benefit of the doubt since they manufactured it. I know it may not make sense.
A few things to consider:
A1 came with a Ubicom CPU. For this reason alone we would never see open source firmware for the router as the Ubicom CPU is not supported in Linux [1].
B1 comes with Linux-based firmware. A statement regarding the GPL was right on top of the router when I unpacked mine yesterday.
How can this be possible if no hardware was changed?
Now consider a typical consumer's perspective, or a support person dealing with most consumers. To them it still has 1xWAN 4xLAN 2xAntenna 1xUSB, LAN is gigabit, wireless is abgn, same form factor, mounting options, etc, etc. To them the hardware ( == visible hardware) is no different.
As for me, I deliberately would not purchase one until I found a B1 in store for the sole reason of wanting 3rd party firmware eventually (not yet avail, but A1 will never have it available). IPv6 support is a bonus.
[1] http://www.dd-wrt.com/wiki/index.php/Known_incompatible_devices