Python 3.0 includes a script that tries to automatically convert 2.6 code to 3.0 code, named 2to3. It's also incorporated into the package setup library, so in theory simply installing a Python 2.6 package with 3.0 will automatically convert the source code.

Of course, that means making your code work with 2to3 to begin with, but that's a much lower barrier to adoption than outright porting and dual maintenance.