Looks like we've got another quasi-Private Practice/Grey's Anatomy in the works -- and the episode during which the crossover appears is already extra special on its own: it's the musical episode! Check out more details on the double-y special Grey's Anatomy musical episode!
As it turns out, Private Practice's very own Dr. Montgomery will be returning to Seattle Grace for an episode -- and more importantly, the musical episode! Kate Walsh, who plays the uber-famous surgeon Dr. Montgomery, describes filming the musical episode as "traumatic" despite going to music lessons.
Also, Kevin McKidd revealed a few songs that'll be featured during the episode: Fray's "How to Save a Life", Snow Patrol's "Chasing Cars", and many more.
Source: Grey's Anatomy Italia