The Lib Dems have not actually agreed to this yet. Both the MPs and executive board (or whatever they call it) have to agree to the terms of the coalition.
From BBC website:
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2215: Senior Lib Dems are still meeting to discuss the deal with the Tories. Nick Clegg needs to win the support of three-quarters of the parliamentary party and three-quarters of the Lib Dem federal executive committee to avoid having to put his plans to a vote of all members.
I agree, however the Lib Dem executive may be a different kettle of fish. Who knows? I wouldn't even know who they are. That said I am aware it is highly unlikely that the coalition will be rejected. I am just one of those people who waits until all the 'i's have been dotted etc.