John's idea of having a compatibility layer sounds like a good idea to me, I'm not aware of the consequences of such solution, but basically, if it is doable, I totally agree it's worth trying as it will help to keep hardcore git/hg/bzr users happy despite the choice we make.
However, I don't think that such a solution should prevent us to make a choice. A choice has to be made, for reasons of common consistence and for not confuse newcomers with a "choose whatever you like" statement. That's worse than picking the wrong DVCS or staying.
What really worries me is that this whole GNOME choice of DVCS discussion seems to be driven as a popularity contest rather than a purely technical decision, pretty Vim vs. Emacs alike. And everytime someone tries to evaluate the tool from other perspective than the hardcore hacker workflow, there's someone with an over reaction, pissing off the people trying to make decisions things on the right way.
There should be a well stablished criteria to make such choice: Which one has better documentation? Which one has a public API that allow us to extend our development tooling? Which one will ease our sysadmins life (bkor, I'm looking at you ;-)? Which one is easier to learn? Which one would ease the transition from SVN users? Which one has positive answers for these questions _now_?
Five years ago, most of us were not hacking on GNOME, and a lot of us won't be hacking on GNOME in five years time either. Making decisions like this based on the personal preferences of the current hackers seems like a really selfish reasoning. Translating or patching GNOME is a task hard enough already, and if we are not careful with our choices in this matter, we would raise the entry barrier a little higher.
If we can make a choice, and still allow compatilibity to other DVCSes tools and workflows, I'm all for that. But despite that a choice has to be made.