On a private server I'm running, we get quite a lot of skeleton traps: in just two days (about 5h of activity per day) of playing on the 47b and 47c snapshots, we encountered about 4 skeleton traps that were triggered, and looking around at the most loaded chunks I can easily find about 5 other untriggered trapped horses.

I should note that we have some command blocks setup in the spawn area that resets the day using "/time set 0" as soon as 1 person on the server goes to sleep.
I'm mentioning this because we're speculating that resetting the time has an influence on the spawning (ie: the day count might be used as a seed to decide skeleton horse spawning) - but we haven't really tested this.