tap tap tap posted (and later deleted) instructions on Twitter that allowed users to enable the "volume button as shutter" functionality via a back door workaround. This is most likely what got Camera+ kicked off the App Store; other apps with "hidden features" or "easter eggs" like this have been banished from the App Store before, like a flashlight app that allowed users to stealthily enable internet tethering.
Apple's well within its rights to react in this manner when developers put "hidden" features in their apps. On the other side of the mobile pond, Android users are currently getting hit by a trojan posing as a media player app which then sends unauthorized SMS messages to premium-rate numbers. While Apple's "user confusion" excuse for not letting Camera+ use the volume buttons as a shutter doesn't necessarily fly (how often are you going to mess with the iPhone's volume while taking a picture?), tap tap tap's "workaround" was out of line, at least in principle -- if Apple's not going to let a flashlight app secretly enable internet tethering or allow apps with blatant malware on the App Store, then it shouldn't come as a surprise that "hidden" functionality, however innocuous, will get your app banished.
Hopefully this ban is only temporary until Camera+ gets updated without the "workaround" in place -- or, better still, Apple could recognize that many users have triple-digit IQs and stop citing "user confusion" as an excuse for denying useful functionality. Either way, I hope Camera+ comes back eventually.