To switch to a background app (multitask) in your iPhone or iPod Touch iOS4 press the square button twice fast:
The four most recently used apps will appear at the bottom of the screen. You can scroll your finger to right and left to view more opened apps and control the music player too.
Apps that are not multitasking compatible will still need to be started again. Some of the apps that I’ve got working on multitask right now are WordPress, LinkedIn or Dropbox. While Skype, Twitter, TeamViewer or MobileRSS haven’t been updated yet.
To close an app tap the app without releasing and wait 2 seconds, then click the red rounded button.