I think you may be right that we may not switch to a single DVCS any time soon, but I don't think it's for lack of a clear winner.
The majority of Gnome developers pushing for DVCS seem to favor git (with bazaar at a distant second). Major open source projects (many related to Gnome) have already adopted git (most freedesktop.org projects, X.org, Linux, Ruby on Rails, OLPC), so there's plenty of experience both using git for large projects and migrating from another VCS.
At any rate, I think there are few people who disagree that DVCS of some kind could improve Gnome development (most interestingly, I think, is the possibility for changes that don't fit nicely into a few commits or a single release cycle).
I'm not claiming that I fully understand all sides of the argument (git is the only DVCS I've used), but the only arguments against git that seem valid are cross-platform issues (which seem to be less significant than they used to be) and migration issues (which we would have moving to any new VCS, and as Federico suggested, shouldn't necessarily be that bad).
I wish it were as simple as just providing mirrors or translation services to let developers use any VCS they like, but any translation between VCSes is going to be imperfect. So using x-to-Subversion really isn't ideal - especially when you're going from a DVCS to Subversion, flattening branches and commits and losing original author attribution in the process.
I know I'm not alone in saying that a migration to git would be a major step forward for Gnome and one that few contributors would regret.
Update 1: link to cgit.fd.o, as suggested by Daniel
A clear DVCS winner for Gnome
June 26th, 2008