The Times today reports on the meeting of Lib Dem MPs that seems to have forced Nick Clegg into a further round of talks with Labour. What is clear is how unhappy a number of those MPs were with the slow movement towards an agreement on voting reform by the Tories. The Lib Dem constitution does not allow Clegg much freedom of manouevre on his own, hence the need to open up a new line of communication, helpfully enabled by the Brown decision. What is certainly true is that the Conservative offer of a referendum on AV would not have been as forthcoming if it hadn't been clear that the Lib Dems had the option of going in with Labour.
The Lib Dems are very divided on who they should go in with, and there is the added complication for them of the numbers - the Lib-Con deal gives a healthy majority in the Commons, the Lib-Lab deal requires an inherently unstable collection of other minor parties to join it. This is difficult territory for the Lib Dems - but it is what they've been wanting for decades.