Like a lot of things online, it turns out that the viral video that singer-songwriter Rob Cantor made of him singing his own song in 28 celebrity voices (the 29th voice was his own) was faked.
Cantor was lip-synching to several people's recorded voices who were impersonating the celebrities. (That said, the impersonations are still solid. And Cantor seems to be able to lip-synch quite well.)
Here's a video that reveals what they did.
Even though Cantor may have faked it, internet sensation Christina Bianco's impersonations of various singing divas wasn't. She even went on to Ellen DeGeneres' show to sing live. (Unless that was faked too. You never know, when it comes to the information superhighway.)