I think that I know the answer to this one but it can not hurt to ask around.
I have a 2000 Mustang with the V-6 engine.
Until yesterday, the engine sounded fine.

Last night though, I decided to tackle the yellowed headlight problem.
It came out pretty good too.
While the headlights were still on the car, I sanded them down with 600 grit sandpaper.
I then polsihed with a drill, a foam attachment and some polishing compound.
Then I used the pad and drill with some hard wax.
These puppies are clear and shine.
(OK, OK...makiing a short story very long...and yes...I know that polishing headlights will not make the engine ping...LOL)

I did also dsconnect the battery and clean the terminals. I also popped open the air intake to see how clean that filter was.

Anyways....my wife took the car out today.

She said she put a few dollars of gas in from a station that we normally go to.

Now the car has a real bad pinging.

My first thought is that it is the gas she bought.
My second was that maybe I loosened a vacum line.
And yes, the timing crossed my mind.

Here is the thing though...
I can sit still and rev the engine all I want. It is silky smooth with no ping.
That makes me think that it is not the vacum or the timing.
I would think that these would makke the car idle rough as well as drive rough.
Plus..I looked around and I do not where I disconnectd anything.

I am wondering though if bad fuel will allow me to idle smooth...while..... from about 20MPH to about 55MPH in acceleration, I have a real bad engine ping.
The power does not seem to be affected and the pinging stops when I am driving along at a steady speed.

It also seems to stop at around 60 MPH.

So what do you think....bad gas or something else?

sounds like a fuel problem. pinging is usually most notable under load which would explain the great idle but rough driving conditions. try to put some good quality gas in it to dilute the bad fuel, remember that pinging could in some cases lead to engine damage.