Here's a resource directory of Python 3 Migration / Porting / Co-Existence information and tools:

David Binger of the MEMS Nanotechnology Exchange writes about their experience migrating their public and private code base such that it co-exists with either Python 2.x or 3.x deployment targets.

The Py3k team has a recommended migration strategy involving the use of their 2to3 conversion package and then maintaining separate releases for the two different flavors of Python. The prospect of maintaining different branches was unattractive, so we decided instead to try adjusting our code so that it would work with both Pythons. As others face this problem, perhaps it is worth describing our experience.

Worth a read. David concludes with "when we do decide to migrate our production systems to Python 3, we think it will be like jumping a creek, not the Snake River Gorge."