At the moment constitutionally only Gordon Brown (as the current PM) has the right to try to form a govt. If he fails (or just resigns) then it'll turn next to Cameron as the cons have the largest number of votes & seats.
Of course, that doesn't stop the Libs & Labour forming a pact but Cameron would probably be PM of a minority government at that point.
At the moment constitutionally only Gordon Brown (as the current PM) has the right to try to form a govt. If he fails (or just resigns) then it'll turn next to Cameron as the cons have the largest number of votes & seats.
Of course, that doesn't stop the Libs & Labour forming a pact but Cameron would probably be PM of a minority government at that point.