I'm not sure how many people are using their camera button for anything but the camera, or even want to use the button for other things. I've got a shortcut to the camera on my homescreen, so I prefer to use it for other things. (However, if I have the camera open, I can still use the camera button to focus and snap a picture.) So, I've been playing with different uses for the camera button.
First, here are two very similar apps that use the camera button to answer the phone -- except when the phone is in use, they won't interfere with the camera function at all.
myLock phone tools - BETA - Android app on AppBrain
Tedd's Droid Tools (beta) - Android app on AppBrain
If you totally want to override the camera button at all times, you'll need this app:
Button Shortcut - Android app on AppBrain
Fire it up, and then assign an app to start when the camera button is long-pressed. Unfortunately, for whatever reason, you'll get nothing but force closes if you try to link the stock Phone app to the camera button... But you could assign a replacement dialer, like this one:
Dialer One - Android app on AppBrain
Whatever you choose, you can still use the myLock Phone Tools or Tedd's Droid Tools apps to let you answer the phone with the camera button.
Here are a couple other ideas I've had for the camera button via the Button Shortcut app.
From this web page: [App] MoreRecent - Shows 15 recent apps (updated 5/6/10) - xda-developers you can download an app called MoreRecent that's just like long-pressing the home button, but better... It displays up to your 15 most recently used apps.
With this app: FastBright - Android app on AppBrain you can have quick access to brighten your screen (useful if you don't like using auto-brightness)
With this app assigned to the camera button: ToggleAir - Android app on AppBrain you end your call with the camera button (since going into airplane mode will cut off the cellular radio) -- then tap it again to bring back cellular service.
If you assign the airplane toggle to the camera button and are using myLock Phone Tools or Tedd's Droid Tools so the camera button answers, of course be careful not to press the camera button to answer the phone until you're sure it will register as a phone answer, instead of an airplane mode activation (both myLock and Tedd's take a couple seconds before the camera button works to answer the phone).
I'm sure there cool uses for the camera button that I haven't thought of... Do add them below.