iOS 4 - Can I close an app without it staying open in the background?
Not presently.

iOS multitasking however is quite different than say the multitasking in Android or Mac OS X. The app unless it specifically is using one of the 7 methods to run in the background is "frozen" and won't consume any resources until it's brought back to the forefront. Essentially the fast app switching feature as part of iOS 4.0 is a large part of why you saw so many apps being updated to work with iOS 4.0 multitasking.