Phillipa, is it just pink or is there mucus too? If it's just pink discharge, I'd call. Unless you've had sex recently, which can irritate your cervix. But if it's thick or mucusy, then it's definitely your plug!
Donna, sweeps really helped move things along with my last 2 babies. The trick though is that my cervix was already dilating and very favorable, and at least with my youngest I was already having some contractions. So in my experience it does seem to make things move along faster, but doesn't necessarily "start" anything, if that makes sense. Still, I'll definitely be getting one (or more!) after my due date. I would like to avoid medical induction if at all possible!