It can't have escaped one's notice that there is a growing body of public opinion which says that swift electoral reform is needed, and I think I even heard Michael Portillo say as much the other day. (Careful, though — I seem to remember a similar surge of opinion which said that the Liberal Democrats were a credible alternative and would do well in the election, and we know how that turned out.) The Lib Dems have been advocating electoral reform since the year dot, and the Labour Party proposed a referendum on AV (the Alternative Vote system) just before the election, although possibly only in a bid to attract the Lib Dems given that a hung parliament was a near certainty.
Of course we have to define what PR means: it's a woolly term which has been used to mean various different systems, some of which are not actually all that proportional. So let me clarify that: I am generally not in favour of (a) any system in which you don't get to vote for a specific (preferably local) person to represent you in parliament, or (b) any system which would have given the BNP 12 MPs in this election. (As an aside, hatmandu seems to have come up with a reasonable solution to (a) while keeping full proportionality with the "Cellular Constituencies" idea.)
What I am in favour of: firstly, AV. Many people think that AV is not going far enough; but it is at least an easy to understand, direct plug-in replacement for the current system in which, unlike the current system, everyone's vote can count (if they rank every candidate). Moreover, as far as the Liberal Democrats are concerned, it seems to me that (a) in a constituency which is seen as a straight Tory-Labour race, Lib Dem voters will no longer have a disincentive to vote Lib Dem because they can still express a second-place preference between the other two parties, and (b) where the Lib Dem are in a strong second place they may well pick up a lot of second-place votes from whichever party is in third place.
And secondly, STV. Compared to AV you lose a bit of local-ness (though in an ideal world the parties will put up candidates from a variety of localities so that you can vote for your nearest one first) but gain in that you might get an MP even if the party you voted for didn't win outright. (In addition, if you are a Party X voter but your local X candidate is a twonk then hopefully they will have put up another candidate that you can vote for instead.)
The people who are going to make this decision are in Westminster, and I wonder if they are even aware that there was a demonstration in Oxford this weekend. The one in London seems to have had more impact, but even there they seem to have been protesting outside the Lib Dem meeting, and if there is anyone who doesn't need to be told about the need for electoral reform, it's the Lib Dems! (Or put it this way: I speculate that a fair number of the protesters are Liberal Democrat voters, and "Lib Dems voters campaign to have Lib Dem policies implemented" is a bit obvious when you think about it.)