I wouldn't rely on the Lib/Dems not doing a deal with a minority government . They will do a deal if it means getting some power and Brown knows this, that's why he suggesting a change in the voting system.
Just look what happened during the failing Callaghan government when the Unions brought this country to its knees , it was the Lib/Lab pact which kept Labour in power for a further 18 months, during which time the country had to endure the Winter of Discontent. We need a strong goverment , whatever its colour, a coalition would fail and we would have to have another election within 12 months.