http://msdn.microsoft.com/en-us/library/system.drawing.graphics.drawrectangle.aspx
FillRectangle, DrawRectangle, FillElipse and DrawEllipse all can take 4 Float (or "Single") parameters: x, y, width, height. DrawRectangle is the only one that will not take a RectangleF, though.
I was wondering if anyone knew why this is. It sure seems like they just plain forgot to overload it.