Will AT&T charge me for using MyWi 4.0 on the iPhone to create a WiFi hotspot?

Yes at&t will charge you extra since you are using an unauthorized app from cydia. If at&t finds out they have the right to add the $45 feature with tethering plan to your account. No if your switch phones using sim cards you will need to call at&t to update your phone so that you can use data plan on blackberry or inspire.