Olympus has posted a firmware update for the 70-300mm f/4-5.6 lens that improves focus during manual focus operation.
Update: Someone asked why a lens would need firmware updates for manual focus. It has to do with the fact that m4/3 lenses are "fly-by-wire." When you rotate the focus ring, you're not physically coupled to the lens elements that are moving, they're controlled electronically. Thus, it is indeed possible to make precision focus more accurate by changing software.