I've thought about it too. It wouldn't be too hard to simply wire in a button that would activate the start function of the ignition, but you would still have to put the key in and turn it to RUN. In order to have true keyless push button start you need much more technology involved. RFID from the fob to the car, a module that cycles through the ACCY/RUN/START positions depending on how the button is pressed, and a few other safety things.
I'm not saying it can't be done (in fact I think someone built a system like this for an Impreza) but to have it fully integrated will be a lot of work.
Edit: Actually, I just did some Google-ing and it seems that you can buy stand-alone RFID secured push button start systems on Ebay, like this one:
http://www.ebay.com/itm/Quality-RFID...-/191098357068
One thing I still have an issue with is that there is no way to control the steering lock, you have to disable it for this to work. In such a case there is literally nothing that prevents someone from smashing a window, putting your car in neutral and pushing it away. And in the case of a manual transmission car, I don't know if there's anything that would prevent a theif from push-starting the car and simply driving away.