One of the few frustrating iPhone related things people have complained about is the fact that the headphone plug is recessed and you cannot use most headphones or earpieces as they will not plug into the device. While I think there is some benefit of it being recessed, it does seem like it was done to drive accessory sales.
The accessory that the Apple Store offers for this issue is made by Monster. I can hear the dollar signs running thought your mind as you read this.
This simple little male / female adapter dongle allows you to use any other 1/8" (3.5mm) jack headphone.
Although not rigid, it is certainly not flexible.
It is a must have if you own an iPhone ... and that is what is frustrating!