So, here I was thinking that some paranoid admin with tin foil hat had configured our Google Apps setup such that the calendar can't be synchronised with mobile devices. A silly idea, since mail can be synchronised.
No, as it turns out, the Calendar app on Android simply doesn't support multiple accounts. That kinda sucks. What sucks even more, is that whatever account you first configure on the phone is the one you're stuck with. You can't just select a new primary: you have to reset the phone.
Grr. Come on Android developers – show me my calendar entries!
*flees, gets coat*