Nick Clegg's comments to Andrew Marr referred to the impossibility of a party that garnered only third place in vote terms being able to hold on to No.10. In effect, they could be in a coalition, but not holding the lead position. This, of course, leaves the door open to a Lib-Lab coalition in which Clegg will be Prime Minister, with Labour, perhaps under a hastily elected new leader, having the subordinate role. What is extremely unlikely is the chance of a Conservative-Lib Dem coalition. Neither party leader could deliver their parties, whose respective grassroots, supported by significant numbers of their elected representatives, viscerally loathe each other. The Tory Party actually thinks Cameron is already too liberal - put him with a genuine liberal, and they would probably stage a coup to oust him.
Will Straw, on Left Foot Forward, in a post that is pretty on the nail about this, considers Nick Clegg as a Ramsay Macdonald, but his analogy is flawed. It is Cameron who would be expelled from his party if he considered holding power with the Liberals - not Nick Clegg. That's how progressive the Tories are!