One of the iPhone accessories that I use every single day of the year is a case that I got for my 4S that has a built in bluetooth headset nestled in the back. It is called the Mogo Talk. The case protects my phone while providing me access to a handsfree headset any time. I love it. In fact, I like this case so much that it is one of the main reasons I have not yet upgraded to the iPhone 5 (they don’t make the case for the iPhone 5).
I charge the headset overnight and then turn it on first thing in the morning. From that point on, I have a bluetooth headset any time I need to take a call or use Siri. The headset is super lightweight, has multiple rubber tips to fit any ear size, and has one button that can be used to answer a call, hang up a call, and to activate Siri (with a 2 second push).
There are a couple of downsides to the case and headset. First of all, it adds about 1/3 more thickness to your iPhone. So, you give up some of the thinness of the iPhone for the convenience of having a headset on hand all the time. The only other downside is the fact that even though the headset charges in the case, it needs a separate charger (included with the case). So, every night I plug in both my normal iPhone charger and the Mogo Headset charger.
There are 2 variations of the case. Both variations fit all generations of the iPhone 4 and iPhone 4S. The only difference is that one partially covers the mute switch on the side of the phone because it had a slightly different placement before Apple fixed some of the antenna issues in the original iPhone 4. The second edition is no longer available but the first edition (the one I have) works perfectly on my iPhone 4S anyways. There is also a case for the iPhone 3G and iPhone 3GS.
Here are links to the two different cases – one for iPhone 3G/3GS, and one for iPhone 4/4S on Amazon: