Despite being heavily involved in Ubuntu advocacy and promotion, being a member of the Ubuntu project, and using it on my personal systems, I've had to keep my servers and development machines running Gentoo.
When developing software, we need to work on what will be "mainstream" in 6 to 12 months, not what's already mainstream. When a new version of our physics library is released, it may depreciate certain methods or include new features we'll really want in our release, but that version won't be packaged until 2-8 months after the new library was released, given the typical release cycles and "feature freeze" stopping new packages from being included too close to release time.
Python 3 is a great example of this; Gnome 3 will apparently use Python 3 for many applets, and Gnome 3's release schedule sets it for inclusion in Ubuntu 10.10 (next Fall), but at present Python 3.1 is packaged but none of the 3rd party packages are available for it - even those which explicitly support Python 3 already.
While doing my periodic Gentoo upgrade, I just noticed that Portage, the package system for Gentoo, can now be built to run on Python 3 directly. When you have both Python 2 and 3 installed on your system, packages wich support both will build and install for both, and their packaged versions are often available just days after release. From what I've seen, Gentoo is one of the best distributions for Python developers right now.
I wouldn't recommend that my family run Gentoo, many of whom happily run Ubuntu, but it's lack of support for developers is concerning to me. I do not believe it should be difficult to both support new Linux users and experienced developers with the same distribution.