It appears that there is a misunderstanding on how iOS handles multitasking. Thanks to Fraser Speirs we now have an extremely detailed report on how it works and what the multitasking bar actually does, or doesn’t do in this instance.
Advice handed out by some people is that you need to kill the apps in order to free up memory. While this can be correct in some circumstances, generally you need to do nothing to preserve the available RAM on your iOS device.
If memory is becoming scarce, iOS will automatically move Suspended apps into the Not Running state and reclaim their memory.
Fraser points out that the multitasking bar (iPad version pictured above) is not actually a list of running apps. Rather, it’s just a list of apps that have recently been used. iOS is able to free up RAM as needed and some or all of those icons listed in the multitasking bar might not have an active app behind them. The bar represents the order in which you last used the apps.
Instead of going over everything Fraser said, I’ll let you go read the full post on his blog which also explains the five states that apps can have.