Poor Cleggy.Convention states that PM comes from majority coalition party.
The bigger question and issue is if Scotland does go, there could very well be a constitutional crisis in that Scotland wouldn't actually go independent until 2016 yet we'd still have 50+ Scottish MPs, mainly Labour, sitting in Westminster voting on UK matters - as well as the 2015 General Election.
And at least that'll give you ammunition for the next 5 years: "Labour only won the election because Scotland wouldn't bugger off"