p.s. - shanks, your bet may require a recount; I was thinking it was either dieresis or digraph, the latter leading of course to the more accurate ligature... but the oddest thing (to me, at least) is that the original spelling of dieresis (and still the Brit spelling?) was diaeresis, with the æ ligature.

That is still the standard spelling here in NZ as well, although the digraph/ligature has tended to be dropped in favour of ae. We spell encyclopaedia, paediatrician and anaesthesia that way, as well. The æ is still acceptable, it's just considered somewhat pretentious, an affectation - probably why I like it so much!