This is a handy little python script I created for randomly placing a number of objects in a user specified area.
Download it from Creative Crash
It has a variety of options for specifying where the zero point for the placement of the objects is, but the most useful I find is the Locator option. This allows the user to interactively move a bounding box around their scene and visually determine the area in which objects will randomly be placed.
The most powerful feature of this tool is the "Stop Intersections" option. This checks the bounding box of each object and checks whether it is intersecting with the bounding box of all the other objects. If it is, then it picks a new random position.