In some office at the Red Sox Foundation. According to a Globe article (Feb 2006):
"Remember that glorious sign? It was the ''Reverse Curve" sign that was traditionally altered by spray paint to read ''Reverse the Curse," for the Red Sox. Once they won the World Series, some flexible person with a can of spray paint changed the tense to ''Reversed the Curse." Then, Governor Mitt Romney blocked Storrow to take down the sign in a photo op, and it was donated to the Red Sox Foundation, although the state had promised this very column that it would remain untouched."
"We're glad it was taken down, for soon the sign will be auctioned or raffled off to benefit Teddy Ebersol's Red Sox Fields, a private-public partnership to rehabilitate the often flooded ball fields at Lederman Park on the Esplanade near the Longfellow Bridge."
But it was never auctioned off.