Argh! ITS NOT A DRIVER ISSUE! Stop telling me its a driver problem when I know 100% it isn't!
Our experiments above suggest that Intel's implementation of shared GL contexts is either borked or, at the least, flaky. The same shared context code works fine on a bunch of other OS/card combos.
Not all GL apps use shared contexts (esp. games) so not all apps suffer from problems like this - which of course doesn't mean there isn't a problem.
I have some ideas for hacks/fixes which I'll try out soon, but in the meantime I'm locking this as it's getting shouty.