Steam has been feeding me some small game updates recently. That makes me wonder if we'll see an event similar to the Summer Sale where we could get tickets and redeem them for prizes. Since during the Summer Sale, a game would receive a new achievement that was checked for the ticket.
If you look at Toki Tori, it has an achievement that states "Coming Soon!"
http://steamcommunity.com/stats/TokiTori/achievements
I've received downloads today for...
Team Fortress 2 - No "Coming Soon!" achievements (it did receive the new "Australian Christmas" update)
Dungeon Defenders - No "Coming Soon!" but it does have a new "Jingled All the Way" achievement
Defcon - No "Coming Soon!" but it does have a new "Merry Christmas" achievement
Toki Tori - mentioned above
Frozen Synapse - No "Coming Soon!" but it does have a new "Now I have A Machine Gun. Ho Ho Ho." achievement
There may be a few games that I don't own or haven't downloaded that have been updated with new achievements as well.