So the HTC One X has the ability to use a Bluetooth headset as a remote shutter. that's great. What about everyone else, though? Fret not, third party software has you covered. Unfortunately I don't have any proper Bluetooth headsets to play with so I can't definitely state that you can get that working as a shutter control, but I'm roughly 99% sure I could if I just got the right headset paired with the right app. What I do have, however, is a bunch of game controllers, so I decided to play around with those as well.
The video above goes through two ways of getting the remote shutter working that way.
One uses a third party camera app called Camera 360, which has the ability to use the volume control as a remote shutter, as well as USB/BT Joystick Center to "convert" the game controller buttons into volume keys. For more info on using USB/BT Joystick Center, check out my game controller guide on Nothing But Tablets, as the same method can be used to get media controls and other functionality out of those controllers. Also, a Bluetooth headset with volume controls might be able to remote control Camera 360 out of the box, assuming that it actually controls the device volume and not its own volume. Unfortunately, I don't have such a headset to test with. If you do, feel free to share the results.
The second method requires root, but works with the stock camera app. It uses the same game controller app, but with a root-only add-on called Joystick2touch that allows you to program buttons to correspond to touch input, which means it works anywhere in the OS regardless of whether an app supports external controls. Using that method, I programmed my Wii controller to control focus, shutter, flash settings, and photo/video mode on my Galaxy Tab 7.0 Plus' stock camera app.
Whether or not this is practical is another matter, but it's a neat trick nonetheless. Also remember that since this latter method emulates touch input, the same basic method can be used for other things where you'd want to add remote control abilities to any on-screen controls.