yeah, totally doable. buy this: http://www.sparkfun.com/products/10748
write some code, do some wiring. victory.
issues? youll be wiring 240vac so unless youre an electrician this is kind of illegal (i wont tell anyone though). also, youre writing code that will control 240vac circuits including heating elements and such so any minor bugs in the code could be highly destructive. no idea how the sparkfun board will accept a thermocouple input - not sure how the analog inputs work so you may need some sort of inline transmitter to turn the thermocouple junction into a 0-5V analog signal. also, i only have experience with 1 roaster (a friend has a gas has garanti) and im not sure how it would take to PID control - the time constant involved may be too large for PID control to be practical over the course of a ~15m +/- roast. what do you do if someone calls while youre in the middle of a roast? using an android over say just a simple arduino or dedicated PID controller introduces quite a few interesting problems.
personally, id be inclined to take a few variables out of the equation - if youre seeking more control over your roasts then perhaps limiting your control system to a purpose built PID module and an SSR would greatly reduce the amount of problems you may encounter and the amount of trouble shooting required. if however youre trying to develop custom roast profiles or something and you are comfortable with the risks involved with using a your code in a high level device to control a large, expensive, potentially dangerous piece of equipment then the android may be a very good tool to achieve this.