I hope you've defriended her on Twitter...shit, down to 294 friends. She'll have to drum up some more. Can't be easy (if it's who I think it is)
PP - Don't go. Namechange, report PMs, tell your nearest and dearest on here who your new name is, and carry on as before. It has all turned a bit nasty on here. If you want to get on and do stuff take a break. But don't leave.
The advent of the Twitter Meta-Mumsnet is not unlike a Moldygate II in the sense that there is now a community, formed out of Mumsnet, not on Mumsnet, discussing Mumsnet, not on Mumsnet. Not in any way a criticism of Twitter or those using it (or FB for that matter which is also a MN sub-community), it is just how it is. I saw a lot of hurt from people on the thread(s) in question who felt they'd been naive/excluded because they didn't realise all this not on MN stuff was going on and being brought back to the site.
However a big danger of extending your MN community into Twitter and FB is that, depending on the extent of info on either of your profiles on those sites, you open up more and more personal identifying information. In no way blaming you but I think it is a further reason why you (justifiably) feel so hurt and well, for want of a better word, ....stalked. This person wasn't just another MN name (love to all the just another MN names out there - we are individuals people - no disrespect) she was someone who might have known where you lived, who you knew, etc and then went to the trouble of dredging up old posts.
You have to report if there is a threat in there. You really do. LeQ has taken Chipping's point on board, and it is true, the only thing we can do is report and get people removed who are going to abuse MN in this way.
Oh, and don't leave. It's the PMer who should be leaving.