Betrand Russell had a theory on Protestant and Catholic atheists. The idea was that a Catholic atheist was an intellectual rebel, breaking away from his firmly established tradition to think freely. The Protestant atheist wasn't - to them, atheism was merely Protestantism taken to its logical extreme.
I certainly consider myself as "culturally christian" despite not believing in God. This wedding definitely emphasised that the culture I've grown up in wasn't a Catholic one though! I'm mildly curious as to rjk's reaction actually, given he *doesn't* consider himself culturally christian.