"In a centralized system, it would be obvious whether the bugfix would have to be backported to the 2.6.34 release; with a decentralized system, it requires inspecting the commit graph. "
How is this true? Not all feature branches in a centralized system need be merged before a release, and so it seems to not be a matter of centralized vs. distributed. Bazaar supports both centralized (ie. non-local commit, merging, etc.) and full DVCS semantics (local branching/merging), and I'm having a hard time seeing how your example would not require graph inspection in either mode, since the commit was written before a release, but merged afterwards according to the comment.
If you simply require all branches in a centralized system to be merged before a release, with non-merged branches to be re-written/rebased off the latest release, then that's one thing. But that can be enforced in a distributed system as well (though it sounds like a painful policy, more painful than whatever it intends to fix).