The esteemed Mr Sheldon Brown argues that crank length should be part of the gear calculation:
http://www.sheldonbrown.com/gain.html

If true, this means that switching to 135mm cranks should give a 20" wheel equivalent gearing to 175mm cranks on a 26" wheel.

Has anyone experimented with this as a supplement/alternative to changing sprocket sizes? I say it's baloney.