Energy Issues ... on my truck

Weird stuff going on with my Ford F-150 pickup last few days. The last few afternoons when I would go to start my truck and drive home from work, it would crank and start fine, but would immediately die at idle. Interesting side note: it is quite challenging to try to drive home on an interstate in rush-hour / stop-and-go traffic without ever letting your engine RPMs drop to a full idle! Coming to a stop required a combo of braking with slight foot on gas to keep engine from dying.

I thought maybe my battery was going bad, but took it in to an Auto Zone last night and they said it was OK.

I also had noticed that the charging system indicator light came on. So I took it to my local garage (Southwest Automotive) this morning, and they explained that that light explicitly means that the battery is fine, but most likely a bad alternator. In fact, they just called me and said the alternator is indeed the problem and needs to be replaced ("only" $450 -- sigh). So of course, what does that warning light that means you have a bad alternator and NOT a bad battery look like?

Like this: