Does TIW expire at exactly 1 year from the date of purchase? Or does it go through the end of the month it was purchased in?
It goes through the end of the month AFTER the year anniversary of purchase. For instance, I purchased one on November 19, 2008 and it was good through December 31, 2009.

One other note - The food there was excellent, and since we had a late reservation we were able to stay and watch Illuminations from inside. And seeing as the temperature had dipped to under 40 degrees, it was well worth it! 
I am now wondering if I should purchase on-line or if I can purchase at the parks??