Developers need to worry about 2.6.26 bugs, presumably, because Linus won't merge your 2.6.27
changes if you didn't fix problems with your 2.6.26 changes in a timely fashion. And your
later work will probably be particularly messed up if Linus gets fed up and reverts some of
the things you did for 2.6.26 because they had problems you weren't taking care of.
I don't think there's really much risk of developers ignoring bugs (that get Linus or Andrew's
attention, anyway) in favor of working on their next thing, since their reputations are on the
line. The more rational fear is that developers won't test 2.6.26, and so bugs won't get
turned up. But I don't think that actually matters too much: I expect that people will develop
against the currently-stabilizing Linus version, and therefore hit other people's bugs, and
people don't hit their own bugs anyway (at least, those that survive to get merged).
One thing that I think would have traditionally been a problem but shouldn't be an issue with
git is the difficulty of preparing a bugfix patch for 2.6.26 when you've been working on a
post-2.6.27 feature. But that's a command or two of setup these days.