Not true. The background services offered by the OS allow the apps to still receive calls/IMs etc while not in the foreground. According to Apple press release:
"These services include background audio, so apps like Pandora can play music in the background, and VoIP, so VoIP apps can receive a VoIP call even when the iPhone is asleep or the user is running other apps."
"These services include background audio, so apps like Pandora can play music in the background, and VoIP, so VoIP apps can receive a VoIP call even when the iPhone is asleep or the user is running other apps."
http://www.apple.com/pr/library/2010/04/08iphoneos.html