So Gordon Brown wants a coalition with the LibDems. Or he may do. He certainly offered some LibDems some government positions. He may have done so expecting them to turn the offer down in order to embarrass them. There is, presumably, nothing personal in it. Brown is said to like Ming Campbell, but knows he has no political judgement and thinks it is worthwhile showing this up.
Why should Brown want to embarrass the LibDems? Well, his thinking probably goes like this. He believes that he can pull ahead of David Cameron, but is not sure his lead will be sufficient to win an overall majority. If the LibDems slip back then any lead (in seats) over the Conservatives will probably be enough to form a government. But mostly, I think Brown decided to embarrass Ming because he can. Playing political games is in Brown's nature.
Campbell, it seems, entertained the offer, even though at one point they were discussing junior positions only with no cabinet jobs. Which goes to demonstrate Campbell's naivete. Joining the government in advance of the election would be a disaster for them. In the Conservative shires it would reinforce the Conservative message that voting LibDem just gets you a Labour government. In the handful of inner city seats (mostly in the north) where the LibDems are battling Labour, it gives them nothing to say. They would probably end up losing both types of seat.
Meanwhile, LibDem activists and MPs are furious with their leader. Unsurprisingly. LibDem MPs hoping to hang on to seats in Somerset, Sussex and West London do not want to be seen as the BOGOF candidates: Buy One middle-aged Scot; Get One Free.