iPhone 4 Not Saving Wifi Password

More information needed. Did you update the iPhone to iOS 5.1.1? What model router do you have, is the firmware on it up to date? Is there another network with the same name in your area? Does the network name have any special characters, and what kind of security does it use (WEP, WPA, WPA2)?

 

I had a similar problem sometime back. My iphone would remember the password to the actiontec router at work, but stopped remembering the password to my linksys router at home. I simply renamed the linksys router, and set it to a different channel while I was at it, and it has worked flawlessly since.

 

Hmm, WEP was never adopted widely as a standard. Even though it works for your iPad and your girlfriend's iPad and iPhone, to ensure compatibility across the board with the latest Apple and non-Apple products, you should consider changing it to WPA2. For other recommended settings for the router, see this.