It appears to me you're running an earlier version of ACR. What has changed on your system since the last time it worked correctly?
Running ACR 6.0. Want a build number? If so how do I get it?
I swear on a stack of PS books, I was using it in a session, I was on the Lens Correction tab editing a photo, then I either hit a key combination or clicked somewhere and it was gone. I thought I saw the screen flicker just before the check box was gone.
So it was within the same session. This happened a couple of weeks ago and I'm getting around to asking about it. Since then I've had multiple reboots.
Running ACR 6.0…
That's an obsolete version.
You want at least ACR 6.2, or you may wish to give ACR 6,3RC ("Release Candidate") a try.
In Photoshop, go the Help menu > Updates… If the auto updater is working, it will take you to ACR 6.2.
Wo Tai Lao Le
After installing 6.2 the check box is back, but this doesn't explain why it went away.
If I had already installed 6.2 and was seeing this symptom what would you have recommended?
Seriously though, you can't just go back a version with a keystroke, as far as I know. The only thing I can think of is that you have two versions of Camera Raw installed, maybe one is 32-bit and one is 64-bit, or maybe a botched installation left two copies in the plug-ins folder. Either way, you're only going to see an old version number if it was never removed properly in the first place.
I currently have CS4 and CS5 both 32 and 64-bit installed, I thought that them being installed on the same computer is a supported configuration. I don't use CS4 and I can remove it.