i wish we could hear more from people making music on the Air.
to elaborate slightly, again, from what i've read, i think the current 13 MBA would do the trick for a lot of music production, and certainly for 'light' work. joeya123's report seems sufficient to evidence that.
the 11 is another story, with its weaker CPU and its much more cramped screen. a part of me longs for the 11, for its size and weight, (and adorableness), and i think one could adjust to the screen, but the screen on the 13 allows for so much more, its hard to rationally choose against it.
the CPU point will hopefully soon be moot, after the refresh so many of us have been waiting for (and others dreading). hopefully either machine will be quite capable, though, if all goes as expected, the 13 will be in a different power class, in which your question may not even apply! ... of course, that remains to be seen.