1.) You force quit an app by holding down the sleep/wake button until the shut down prompt comes up, then hold down the home button.
2.) There is an iStat app for iPhone.
Oh, and all App Store apps quit upon pressing the home button. ALL of them. All you have to worry about is Cydia apps, iPod, Mail, Phone, and Safari. But Phone and Mail are going to run in the background no matter what.