Today I noticed, while checking to see if iOS 4.3 was avaialble yet, that the iPhone was not detected by the Mac. No detection, no updates.

I checked in About this Mac -> More info -> USB and the iPhone was listed. So what went wrong?

I do not know the full answer, but it may well be caused by installing the iPhone SDK, which I updated recently. Fortunately the fix is simple. Download and install the iPhone Configuration Utility. You do not even have to run it; it fixes up whatever outdated or corrupt files caused the problem.