Sorry for my post not being informative and on topic, but I just want to say, that if you pull this up I will personally make sure that those Arduino bashers at avrfreaks.net will apologize in public here. They have this thing about Arduino guys:
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=87838&postdays=0&postorder=asc&sid=564a39edce276b9f3e435d83793d809f
And on topic - have you researched some PID algorithms? There is a nice PID library for the Arduino...