It is not a multitasking list - it is a fast app switching and recently used apps list. All apps in the recently used apps list are not running in the background.
The overwhelming majority of 3rd party apps will continue to be quit when leaving the app, but the app will be appear in the recently used apps list.
Some apps such as games or with any app that a developer wants to make use of it will be in a suspended state with leaving the app so when returning to the app you can begin where you left off instead of having to begin from scratch. An app that has been suspended when leaving the app is not using any processor cycles but will continue to hold the RAM or memory it was using before leaving the app.
Some apps such as a music streaming app or a GPS turn by turn voice navigation app that has been updated for iOS4 to make use of it will continue to run in the background when leaving the app.
http://whenwillapple.com/blog/2010/04/19/iphone-os-4-multitasking-explained-agai n/