I am absolutely sure that background apps would be optional. Every app is optional.
You cannot build a good IM app or a good calendar app without the app running in the background. Without that, the IM app would never notify you that you received a message, and the calendar app would never remind you to go to your appointment, unless you actually took out the iPhone and launched the app.
A calendar app would use approximately zero battery power - it doesn't have to send any signals or anything. An IM app would have to use the internet, so that would use some battery. And what if you had SETI at home running as a background app? That would probably eat the battery AND slow down performance. That is why nobody would download SETI at home, the iPhone app. That would be a hilarious iPhone app.
My point is, if the support for background apps was introduced, people could pick and choose which apps they wanted running in the background based on how much they need the app and how much it would shorten battery life.
I doubt that merely introducing support for background apps would affect the battery life in a significant way, only INSTALLING such apps would have an impact.