(Not an answer but my comments are too large for a "comment")
This is more computational geometry than graphics. First of all, it would help if you could give an example (a drawing) which describes your input and your expected output. Are you working with low count polygons, or precise coastal maps? Do you want a real-time solution or is it off-line? It could be a big help if you would describe what you want to obtain with these rectangles covering your polygon. Also, what are your needs concerning the solution(s), for instance do you want the area of the rectangles to be as least as possible?
Intrinsically this is NP-hard, this means there is a vast amount of possible solutions when the size of input data rises. Think of a circle covered by a square divided in two not necessarily equally sized rectangles. There is an infinite set of solutions already there concerning where they are split. Then just think of the amount of possible solutions for optimally packing a star shape into five or so rectangles, it's simply "not possible" if you want it done on a computer, in finite time.
So, you will be looking for a heuristic, that gives you a solution but not necessarily an optimal one. This means that you need to figure out what you want to accept being a solution.
Edit: One variant of the problem at hand, that I didn't think of, could be that you already have N rectangles (size is given up front, they might be of the same size) that you wish to cover a polygon with. It's not simpler, also it could be impossible, for instance if the total area of the rectangles is less than the area of the polygon. So: Please describe your problem at bit more :)