Simple mathematics; 39 week between September and May, 24 episodes of Grey's Anatomy ordered for its fifth season -- that means that Grey's Anatomy will either be preempted or show a rerun during 15 weeks between September and May. As it is, there will be a new episode this coming week -- really at least for the next three weeks.
They've already broadcast 16 episodes, and with these three new episodes in a row, that'll be 19 episodes broadcast by the end of the month. There are either seven or eight weeks between the beginning of April and the end of the television season (depending on which day of the week the end of the television season is on), so there will have to be two or three more reruns before the end of the season. Expect them mid-April.
I hate reruns of Grey's... This is the one and only show that I really get into right now. When it's a rerun--it depresses me..lol I like to escape into the show but can't do that if I have seen it already.