Why is my iPhone not 'seen' for no apparent reason when I connect it to iMac? It's always worked before.

The 'why' I can't answer. Applecare rep was unable to help either. I have an iPod and sync worked fine on that.

My solution. I erased all the data on the iPhone. I tried to sync again but to no avail.

I quit iTunes and downloaded it from the website and installed it.

When I connected the iPhone I was asked if I wanted to connect it as a new iPhone or sync from old backup. I chose the latter.

Hey presto; everything is back to normal!