There is one major thing that the Lib Dems and Tories have in common - they both hate Labour, so they might be able to agree on a PR system that hit Labour the hardest.
PR is anathema to the Tories as no system could seemingly benefit their party, which is why they favour gerrymandering the current system instead. But the Lib Dems are not asking for PR, they are asking for a REFERENDUM on PR, which is not the same thing - a referendum could be lost and if the Tories have a say in the system and the wording of the question they could achieve a rejection of PR more easily.
Publicly, the Tories are loudly proclaiming that there is no clamour amongst the public for a change in the voting system. In which case the Tories would win a referendum so why their panic and hysteria? They have no moral argument for resisting giving the people a say. In practise of course, the Tories know full well that the public do indeed favour change and are not so sure they could turn this around, even with the mass press on their side.
The Tories are still hopeful they can scrape a majority of seats under the present boundaries. But after this last election where seemingly everything was in their favour and they still fell short, we have to wonder. Only another hung parliament after this one might change the Tories minds on electoral reform.
Even if the Tories do manage to change the boundaries, how long could they hold out on reform. More enlightened Tories might sense it is better to agree a system with the Lib Dems now that benefits them, rather than risk a lib-lab PR compromise that really hits the Tories hard. What might the Lib Dems and Tories favour?
A 3 seat (single-transferable vote) STV, or even better, the arrangement they use in Ireland of 5 seat urban and 3 seat rural is the best way to hit Labour prospects. To win seats in rural areas you would need to reach 25% of the vote, the Lib Dems and Tories can manage this easily but Labour would struggle. While in 5 seat urban constituencies, the Tories would benefit most from a 17% threshold where their vote is very weak.
The Lib Dems like STV because it still keeps out smaller parties but does not harm the third place party. The Tories might find this STV system even more attractive if it was accompanied by larger constituency boundaries - a gerrymander that would help them significantly.
A PR deal with Labour however would probably lead to a mixed proportional system with a much lower threshold - maybe 5% - this would hit the Tories much harder.
Even if PR for Westminster was ruled out, the Lib Dems might still be able to sell a deal to their members if they could offer PR for local government and a PR elected second chamber. Coupled with more devolved power to local government, a written constitution and an elected second chamber using a PR system - this might be sellable. Whether the Tories would even concede that though is debatable. Personally the Lib Dems will have to have a contract written in Cameron's blood for anyone to believe that as soon as Cameron got the keys to number 10 he wouldn't double cross them. We shall see.