Do you have any apps that use the camera button for something else? I'm thinking specifically of that VF Dialler thingy that uses the camera button to initiate voice-dialing.
And, of course, try a reboot -- maybe that'll fix your problem.
For anyone looking for a long camera press to voice dial...
Searching the internet landed me on this page,
The Droid and hands-free voice dialing | Android Atlas - CNET Blogs where I learned it is called Voice Dialer HF and is $4. The Dialer isn't what takes over the camera button's long press. Something called Button Shortcut, in the app store for free, does. You can assign any app to a long press of the search button or camera button. I was hoping a long press would unlock the phone as well as initiate the voice dialer but it does not. You still need to wake up the phone and then slide/unlock the screen. VD HF is now the 2nd app I've payed for.