You could just install the new version alongside the old one, and simply use the new python where needed. I had an "oldish" installation of Python at work and since I didn't have the permissions to upgrade the system, simply installed a local (newer) version and used that. If you download the package, there are instructions for that. You'll need compilation tools for that, of course. I'm not completely sure how it would work out with the binary packages, maybe somebody else can shed light on that -- if and how it's possible to install a binary package to a non-default prefix -- but if you have the compilation tools ready, it's no hassle installing it to a different prefix, for example $HOME/python-2.6, or /opt/python-2.6 or where ever you want it.
Of course you can just check if some Debian reposities have the 2.6 version of Python (or version 3) and use apt-get to upgrade the installation, but don't be afraid to compile another version either. Just grab the needed compilation tools from Debian reposities and get to work; if I recall this right, it's just a matter of passing --prefix=/new/prefix
option to the configure script and then proceeding as usual.
See the 2.6 release page
at python.org for more information (download the source and read the README file). Or if you're interested and it's ok not to use version 2, click the link for 3.1.1.