Indeed, the version of Visual has to have been built for a specific version of Python, and a mismatch cannot work. Just this week I worked a bit with Python 2.6 to see what the issues are going to be. There are some problems. The Boost libraries, which enhance C++, have to be built for a specific version of Python, and since there aren't yet available these binaries for 2.6, I built them from source. Similarly for numpy. Turns out that something has changed in these components, because there's now a type mismatch between numpy integers and Python integers (this is a simplified description). Moreover, for the new Visual about to be released, there's a problem with a native-mode version for the Mac because the Mac Python 2.6 has an IDLE that won't run!
Summary: There are various problems in the Python 2.6 world. But we'll be tracking them and looking for solutions. We would prefer to release the new Visual for Python 2.6, because the new Python builds a bridge between Python 2.5 and the coming Python 3.0, a version of Python that will actually break some existing programs. Python 2.6 offers tools to check on possible incompatibilities.