Tories: a referendum on Alternative Vote (AV) - with the Tories whipped to vote in favour of the necessary legislation, but the Tories campaigning against AV in the referendum.
Labour: a bill to deliver AV without a referendum. A referendum on a PR voting system (either AV+ or Single Transferable Vote - it hasn't been decided yet). Not clear whether Labour would campaign for PR in the referendum.
So a coalition with Labour would present a much better chance of bringing about AV. But AV, of course, is not proportional representation - so would the Lib Dems go for it? Some of their present successes under FPTP elections rely on the rest of the vote being split between Labour and the Tories (and sometimes others like the SNP or Greens). Analysis by the Electoral Reform Society suggests that AV would have given the Lib Dems 22 more seats if people had voted the same way they do now under AV. But of course that's meaningless, because huge amounts of voting is tactical - particularly in marginals, and a high proportion of Lib Dem seats are marginal. Under AV, you wouldn't have tactical voting, and so the vote counts might be completely different. It's not at all clear to me whether AV would help the Lib Dems that much. Whereas proper PR almost certainly would help them.
So, if I was a Lib Dem MP I'd be very tempted by the Lib/Lab/plus "traffic light coalition" option as the only route to a voting reform that could actually offer my party anything substantial. However, I'd insist that as well as introducing AV, Labour campaigns for STV (or AV plus) in a referendum to be held before the next election. Whether STV or AV plus could be implemented before the next election is unclear - apparently the boundary changes would take at least eighteen months to implement because of legal shenanigans. Assuming that a referendum could take place in early 2011 at the earliest, could the traffic light coalition last until - say- 2013 to deliver the necessary reforms and wait for the boundary review? It'll be damned difficult.
But why do we need complete boundary changes in the short run? As I understand it, STV operates in multi-member constituencies (as does the current system for the European Parliament, which surely should be considered as an alternative Westminster model - simpler than STV in many ways, and probably my personal favourite.) So if you have 5 MPs per constituency (for example) it's easy: you just aggregate up the existing 650 constituencies into 130 multi-member constituencies. (might need to do some fiddling around to preserve national integrity -i.e. no constituencies overlapping between England, Scotland, Wales + N Ireland. But not that hard, surely?) In the longer run I'd recommend equal sized populations for constituencies (ironically what the Tories are advocating now, albeit just so they can re-rig FPTP to get a semi-permanent majority). But a modified version of current constituencies will do for now. I think this could be in place for an election as early as summer 2011.
There are rumours that by the end of the day we'll know whether Lib-Con or Lib-Lab+ is our future... but then the lobby said that yesterday, and the lobby was talking total crap, so don't hold your breath.