Maybe this is a false choice. Maybe the problem is standardizing on Debian stable. If that distribution isn't giving us and our users what we need, maybe we need to re-evaluate that choice. Yes I know we've talked about that before and yes I know it would not be easy to switch to something different, but still. If you can't even upgrade svn to 1.5 on Debian stable, then I think you'll find it impossible to switch to any modern DVCS.
Maybe. Again, we should separate between client and server. The server we can control, and adjust as needed. The clients we can't (heck, we even support Windows :-)
If "switching to a modern DVCS" means that users now need to start compiling their VCS before they can check out Python, I don't think we should switch to a modern DVCS. Such a system must be mature, and if it isn't included in Debian stable, it can't be mature (and free software).
In the specific case, if a decision is made to switch to bazaar, and bzr 1.5 is recent enough, then I'd be happy to upgrade to testing (although 1.5 is also available from backports, and already installed; stable has *bzr 0.11*). Since lenny was frozen, bzr managed to release 5 minor versions (so it is 1.10 now); this makes me very worried whether this software is mature.
IOW, Python shouldn't require a VCS that is not even a year old (a year ago, bzr 1.1 was released).