Short version - my (slightly) child theme of boost worked properly in 3.2 and doesn't work in 3.3. The most noticeable thing is that the burger icon is broken.
My version...
...and inspecting the HTML
Switching to the vanilla Boost theme, the markup is different...
It's a font-awesome thingy.
Now, I don't have any templates overridden or any custom renderers. I don't even mention the menubars, fa-bars or anything like it in my theme. I'm really struggling to see why it is generating different markup. Any thoughts much appreciated.