Doesnt Gordon Brown remain PM if it is a hung parliment?
Normally the party that finishes with the highest number of MPs will be asked to form a coalition government, so if Conservatives finish first, Labour second and the Lib Dems third, it'll be the Conservatives and Lib Dems in government with Cameron as PM. Naturally the Lib Dems will want something in return for being in government, and so they could refuse to go with the Conservatives and form a coalition with Labour instead.