I think the major sticking point with the tories is the electoral reform thing. Without that they are likely to walk and go labour... and now that Brown has resigned (which amusingly I have seen described in two different headlines as 'a big mistake' and 'an astute political move' ) they are more likely to talk to Labour seriously about a coalition.

The other option is a three way split - with tories running a minority govt and the Lib dems/labour either opposing them or agreeing with them. I think the last time this worked was one of the world wars, though, because the only real policy any party had was 'fight the germans' so they agreed on everything