According to numerous rumors all over the web, the next episode - My Favorite Mistake - which was originally supposed to air on March 29, may actually air on March 22 - this upcoming Thursday! According to these reports, ABC wants to keep the Grey's lead-in on October Road, since it did MUCH better than Six Degrees when it premiered. However, nothing yet has been confirmed by ABC. Although the official website says it is airing on the 22nd, ABC Media Net is still reporting it's airdate as the 29th. Stay tuned for more updates on this!
(Also - My Favorite Mistake is not the spin-off episode as previously reported. That episode, which has yet to be titled, will be two hours long and is currently set to air on May 10th.)

UPDATE [6:54 PM]: A source on the ABC Message Boards e-mailed ABC to ask them, and here's the reply they got back -

The episode was originally scheduled to air March 29, but it should air on the 22nd now.

Best,
Amy Astley
ABC Media Relations

So, there you have it. My Favorite Mistake will be airing on March 22, 2007.