see a detailed description at stackoverflow http://stackoverflow.com/questions/14495111/setting-hex-bins-in-ggplot2-to-same-size, in the lower part of my answer (http://stackoverflow.com/a/14586941/755257) you'll find reproducible examples.
My guess is that hexBin should produce width and height analogously to StatBin2d yielding all four corner coordinates for each bin, so geom_hex knows what it should draw. Incidentally, this would make geom_hex more general.
Unofrtunately, at the moment I don't have time to produce a patch.
Thanks a lot,
This sounds like a great feature, but unfortunately we don't currently have the development bandwidth to support it. If you'd like to submit a pull request that implements this feature, please follow the instructions in the development vignette.