I guess the way I see it, the Conservatives are close enough to a majority on their own that they wouldn't be prepared to make very many concessions to the Lib Dems, and any that they do make I can see them whipping away when it suits them.
As much as I can see that a Lib/Con coalition holds more "legitimacy" in a sense than a rainbow coalition, I just feel that siding with the Conservatives will do nothing other than alienate a generation of Lib Dem supporters for nothing. Any referrendum on electoral reform will come at the price of a huge negative campaign from both the Tory-led media and official Conservative policy announcements, which will set the potential for PR back for goodness knows how long. And while that might have been an acceptable loss if the country got sorted, the fact is, Liberal and Conservative plans for the economy differ so wildly that I can't see the coalition playing nicely. About the only thing the two parties agree on is that education needs looking at, and even there the plans are different, with Conservatives wanting to hearken back to 'core' subjects.
The coalition would fall apart and look bad for the Lib Dems and coalition governments in general, and the Conservatives would push through their most dearly-held plans anyway. I can see that being the 'big society' malarkey, which would leave the poorest in the lurch while the rich get along fine.
For me, a Lib/Lab/rainbow coalition holds no less legitimacy, and would lead to a far better future for Britain. The Lib Dems would have a greater amount of power, and so could cut the Labour red tape that has built up over the years, and it would be in everyone's - even Labour's - interests to show that the coaltion could and would work, which would be a far better argument for PR and a successful referrendum.