Thanks for the reply, Foodeater! I didn't cook anything acidic in the stock-pot last night--just a simple chicken and rice dish. I called a local shop that does tin re-lining this a.m. for advice. They said that it might be hard water stains. We have very hard water (we've replaced two kitchen faucets in 3 years) so that might be the culprit. I let the pot soak in soapy water for about 2 hours last night before finally cleaning it out (we had an unexpected guest).
The kitchen shop said that there should be no damage to the tin from a cooking perspective but it just looks so unsightly. Maybe I will try the baking soda technique. Sigh... If anyone has any more ideas, keep them coming. Thanks.
"Live! Life's a banquet and most poor suckers are starving to death!"