The CW's "Life Unexpected" has a premiere date, Jan. 18, where it will temporarily replace "Gossip Girl" as part of a CW strategy to keep original episodes airing on Monday nights.
"Unexpected" will take the place of "Gossip Girl" for its run, then "Gossip" will return for another 12 episodes.
The logic is that "Gossip" performs stronger when aired with fewer interruptions. Last year the show dropped significantly in the ratings during the early part of the year as the network cycled in repeats.
via our friend James Hibberd at THR.com
Not sure whether that "logic" is the CW's or Hibberd's, but how a repeat hiatus vs. a replacement hiatus is going to be any different for Gossip Girl doesn't quite make sense to me. And as I recall, it seemed to me that Gossip Girl went into repeats to prevent from getting pounded into ratings dust by new episode competition in its (last season) 8pm Monday timeslot.
Update 2: After that, it is expected that Life Unexpected will move to One Tree Hill's spot and that between the three shows CW seeks to reduce the number of low-rated Monday repeats.