While I have not yet upgraded to IOS5 (some of my apps won't work with the new notification system properly, I'm looking forward to the Custom Contact Vibrations feature that has been added. This feature is similar to being able to add custom ringtones or sms message alerts to a contact, in that you can have something specific to one contact so you will be able to identify who is trying to get in touch with you.
Essentially you get to tap out your own rhythm and the iPhone saves it and replays it for your contact when your phone is in silent mode.
This LifeHacker article will help you set up Custom Contact Vibrations.