Doesn't 104.7 The Fish do all-Christmas in Atlanta still?
In Detroit, 100.3 WNIC (Fresh 100) switched back immediately at midnight on 12/26, and WMGC (Today's 105.1) waited until noon to switch back. 105.1 continues all-Christmas on its HD2 (which is normally an '80s hits format). 101.5 The River in Toledo, OH kept mixing in some Christmas songs with the regular music, but they appear to have dropped that.
Personally, I think any and all songs mentioning Santa Claus immediately become obsolete and irrelevant at midnight on 12/26, and I don't care to hear them again until the following year. I don't, however, mind hearing (as A.J. said earlier) general "wintertime" songs that don't mention Christmas in the lyrics (such as "Sleigh Ride," "Let it Snow," "Marshmallow World," or "Winter Wonderland") or religious Christmas carols after the 25th. From my experience, it's the contemporary Christian stations that do the best job of keeping the real feeling of Christmas going after the big day itself is over, because they tend to stick to the Christian carols and also non-carols by Christian artists who don't get played on secular radio. Although I did hear one CCM station play "O Come, O Come, Emmanuel" today - being an Advent song, I think that one has outlived its usefulness until next year.