They re-broadcast episode #501 on December 14, and they re-broadcast episode #502 on December 21, so episode #503 was the next episode ready to be re-broadcast.
There are 24 episodes going to be made this season (two more than Season 1, same as Season 2, and one more than Season 3), and 39 weeks between the premiere on September 28, 2008, and the finale, expected on May 17, 2009. That means that on 15 weeks during that period they will either have to show a re-run or pre-empt the program.
Since they had 11 episodes left to re-broadcast (before last Sunday night's broadcast), and 17 weeks left to broadcast episodes, 6 weeks will need to be re-runs or pre-emptions. They cannot re-broadcast or pre-empt episodes in early-/mid-March or in May, due to Nielsen sweeps.
Why this past Sunday instead of some other Sunday? It is hard to say. Clearly, this coming Sunday will be a re-run (because of the Super Bowl), so that puts two re-runs in a row. That tends to be better, because viewers tend to hate the "one week a new episode; the next week a re-run; then another new episode; then another re-run" routine. I think it is also pretty-well understood that hiati of more than three weeks tend to hurt programs (with specific exceptions, shows like Lost, Heroes, and 24). So given that they had to do 6 more re-runs (or pre-emptions), they're probably aiming to do three sets of 2 re-runs in a row. The first is obviously last week and this week. The second pair will probably be right after February sweeps (which is in March this year, so figure late-March for the re-runs). The final pair probably right before the end, so figure late-April. I'm not sure though.