Posted this in another thread, but here's how the rejoin feature appears to work:
It appears Blizzard is using a modified version of the replay feature in StarCraft II to get players to reconnect to a game.
The time it takes to reconnect takes longer, depending on the duration of the game you were in. Additionally, you can hear many of the triggered sound effects which occurred during that game playing quickly in the background behind a reconnect screen. This leads me to believe that behind the scenes, the replay feature is fast-forwarding everything which transpired in that game at 16x normal speed (the max FF speed of the replay feature) until it catches you up, then drops you back into the game.
It seems quite possible that if they are using the replay system from StarCraft to synch the code among players, it will make it into StarCraft 2. You can already watch a replay with your friends over StarCraft II and jump into an old game at any moment, as any player.
We've used this many times to find bugs in SCU, and the fact that it actually works in SCU means it will probably work for any custom game.