'Grey's Anatomy' and its spin-off, 'Private Practice,' are at it again with their crossover episodes.

The Ausiello Files is reporting that Walsh will come back to 'Grey's' in early January for a top-secret plot centered around Addison, Mark and Leven Rambin, who recently joined the cast from 'Terminator: Sarah Connor Chronicles.'

Apparently, Addison returns to Seattle at Mark's request to perform surgery on Leven's character. But, as executive producer Shondra Rhimes spoils, the plot will extend back over into 'Private,' when a complication has Mark transfer the patient back down to L.A. again (so much for the plot being "top secret").

News about the Grey's Anatomy spinoff have been pouring in since late February when it was announced that Grey's character Addison would possibly get her own show. Since then, fans and critics have been speculating as to whether this was a good or a bad idea.

Earlier this week, ABC gave official details about the two episodes that will serve as a test to see if the spinoff will indeed be part of their 2007-2008 lineup or if Addison will stay at Seattle Grace.

Even if the two-hour spinoff episodes will only air on May 3, we can already start speculating as what series we will like better: Grey's Anatomy or Private Practice (the spinoff's tentative title).