When I first read this, my first instinct was "don't", and I think that still holds. You are mixing water, electricity and possibly natural gas in a single untested design, and if anything goes wrong your pool could drain out, you could get electrocuted going in the water, or you could even end up with a bomb. It's not worth whatever costs you'd save building your own heater versus buying a commercial unit.
If you are really serious about a DIY pool heater, I think the best (safest, most reliable) route would probably be a heat exchanger sourced from a commercial hot water heater tank. You can build the heat exchanger yourself with a 55-gallon drum and some copper plumbing pipe, but for a whole host of reasons I would buy the HWH commercially (you can get gas or electric; your choice, but either way the HWH should be kept in an enclosed indoor area and properly connected to your home wiring/gas). Then, simply draw water from your pool into one closed loop of pipe in the drum, where it will be heated by being in close proximity to the loop which is fed by the HWH (a transfer medium like more water between the two coils might be a good idea). The HWH's water can feed back into itself, and due to natural convection, as the water is heated by the tank and then cooled in the exchanger it will flow through the loop, so there shouldn't be any need for a pump on the HWH side.
The tricky bit will be the thermostat; you'll want the water from the HWH to be hotter than the desired pool temp so that you can rapidly heat such a large volume of water, so you'll need a method to monitor the ambient pool water temperature and stop running water from the pool through the heat exchanger. I think the easiest design would be a temperature monitor that controls an inlet valve, and then the inlet valve is simply a diverter that draws water out of the main circulation pump's loop. If you design the exchanger right, convection can work for you here as well, or you can use a Y with a reducer off of the circulation loop to let the pump's pressure help you.
If you simply must do it all yourself, instead of introducing heat through a water coil from a HWH, fill an inner drum (40 gals or so) with water, put the loop from the pool in that, and then put that in a larger drum with some gas burners or electric heater coils underneath. When the pool needs to be heated, turn on the burners; that will heat the standing water in the drum which will then heat the water flowing through the pipes. You will need the same thermostat with will control both the flow of pool water and the heater element, and you'll also need a thermostat and a temperature/pressure relief on the inner drum (for a demonstration of what can happen without it, check this Mythbusters clip: http://www.youtube.com/watch?v=rGWmONHipVo)
Whatever you do, do not heat your pool water directly. First, it will be very difficult to control how hot that water gets, which can result in burns if the heated water is returned directly to the pool without getting a REALLY good chance to mix with ambient water. Second, there are a lot of chemicals in your pool water, which can react with the water line under high heat (like an open flame), and can erode it causing your pool to drain (and if you're using a gas burner, the flame could go out and then you have a gas leak). Commercial pool heaters are designed with multiple levels of redundancy to prevent overheating, flameout, or energizing the water with any electrical components. You will need to provide similar protection in any design you come up with yourself.
