Hehe, all my first three posts on this forum is related to pimping the 12" PB and I must say it rocks! I do have the Rev A one so the Rev B can only be better. They fixed the heatproblem (which I don´t really see as problem) and the Rev B also has DVI and a better graphics chipset.
As for the formfactor, the 12" is the right size if you want to carry it around a lot. 0.5-0.6 Kg may not seem like much but after carrying around for an hour or more you wish it wan´t there. You basically get all the features from the larger models except clockspeed (1 Ghz 512 kB cache is enough for most things anyway), memory (unless you´re a serious movie editor 1,25 GB will be enough as long as the rest of the machine keeps up) and resolution/screensize. The resolution is a bit of a turnoff sometimes and yes, sometimes 12" is slightly too small but i´ve done lighter photoshopping in without trouble and with general use, it´s not a problem.
I´d say go for the 12", you won´t be disappointed =)