you can take your OEM wheels and tires to a shop and have the sensors taken out, depending on the shop, could cost you $40 a wheel. Then have the sensors installed on your 20" wheels, another $40 dollars... you see where I'm going with this?
Some places, like Discount Tires, sells manufactur specific sensors. You can have your new rims, tires, and sensors mounted and installed for delivery. However, you'll need to take your new shoes to the dealership and have them reprogram your car to read the new sensors. The sensors in your OEM wheels will no longer work with your car (unless you go and get car reprogramed, again)
Another option that I've heard of (use at your own risk): Remove the four sensors. Put the sensors into your spare tire and inflate the spare to the proper 32psi or so. The sensors will be reading the correct pressure at all times, no matter what tires/wheels you have on the car. But then again, if one of your 20"s is running low on air, you may not know about it until it's too late.