Oh man.

I can't believe I am having this debate:

From: Clay
To: Big Daddy
Subject: RE:

Springfield, Vermont??

http://www.usatoday.com/life/movies/simpsons-contest.htm?loc=interstitialskip

From: Big Daddy
To: Clay
Subject: RE:

I always figured it was Springfield Illinois because of it being the capital of Illinois and the reference to 'Capital City'

From: Clay
To: Big Daddy
Subject: RE:

Springfield isn’t Capital City though… that’s somewhere they visit… a big city… I always thought it was Springfield, Mass. It’s big enough to have a minor league team (the Springfield Isotopes), and “Capital City” would be Boston.

From: Big Daddy
To: Clay
Subject: RE:

yah, i know, but it has always been a running gag that they never say what state it is and 'capital city' is a subtle hint from the producers