AIM on iPhone: Frequently Asked Questions - Discover AOL
Do I receive IMs while I am using other applications on my iPhone or iPod Touch?
By default, if you exit AIM and return to the home screen, AIM will maintain your session for approximately five minutes. If you return to AIM within that time, you will receive messages that were sent to you during that time. If you do not return within five minutes, you will be signed off from the AIM service and will not receive any messages sent during your absence. A future version of AIM will be able to notify you when you receive messages while using other applications.
What does Sign-off on Exit do?
Choosing Sign-off on Exit will cause AIM to sign you off of the AIM network when you exit the AIM application. By default, AIM will leave you signed in for about 5 minutes after exiting AIM and will queue any messages received during that time and deliver them if you return within that period.