Selecting a compatible BB: spindle offset Replaced Shimano Bottom Bracket, now front derailleur won't shift to largest gear
How do I measure a bottom bracket?
I thought the only thing that mattered when replacing a bottom bracket was the thread size (68mm) and the spindle length. - unsavory

Me too. I had thought that the things to consider are:

Thread specification e.g BC 1.37 x 24, English or Italian
Thread width: 68 or 73mm
Cranks interface
Spindle length e.g. 113mm

And now it turns out the axis offset matters. How do I determine this before purchase. E.g. in this datasheet there is no mention if the axis is symmetrical or offset.
When should I be worried about unsymmetrical BB spindles and where should I check to ensure compatibility?
With Shimano there are "E" and "K" variations of bottom brackets that refer to coaxial mounted front derailleurs and chain guards respectively. The BB variations provide for enough space on the drive side spindle to build in another "ring" of the derailleur or the chainguard carrier.

For exact paramenters Sheldon Brown could be helpful too.
http://sheldonbrown.com/bbdrop.html