Camera+'s developers tap tap tap must feel my pain, because they recently added this much-requested feature to a new version. Unfortunately, Apple rejected it from the App Store, because repurposing the volume controls could "potentially result in user confusion." Tap tap tap is philosophical about this decision, saying they understood Apple's position and that they hoped they would change their minds in the future. Although a few other camera apps do offer the use of volume control as a shutter release, these were either only available via the jailbreak store or the developers smuggled the feature through under Apple's nose.
However, it turns out the current version of Camera+ (v1.2.1) has this feature, albeit hidden away-- but it's a snap to activate (pun intended).
(Thanks to Dan at UneasySilence and everyone else who sent this in!)
Just fire up your Safari browser and enter the following URL into the address bar: camplus://enablevolumesnap. Hit the Go button and you should see Safari close and Camera+ load up, except now when you go into the camera mode hitting either volume button will immediately take the picture. Should you wish to turn the feature off again, simply repeat the process but with camplus://disablevolumesnap instead.
It remains to be seen what Apple will make of this, if anything. Whilst this is a pretty innocuous feature to have snuck into a shipping piece of software, it's still a big no-no to knowingly include hidden features that would cause an app to be rejected if they were out in plain sight. Consider the flashlight app with a hidden tethering mode -- according to the absolute letter of the Developer Agreement Apple make you sign, Camera+ v1.2.1 is no more legitimate than that.
Clearly, though, that flashlight app was directly costing AT&T and other operators money, whereas the worst this can do is confuse anyone who somehow manages to enter that URL and then completely forget they've done so. Apple's next move will be very interesting to watch, particularly because many users (myself included) have been begging for this feature since they first touched an iPhone.