Related note: Michael Jackson's brothers are talking up a tour and they mentioned possibly having a hologram of Michael for the shows. The idea was discussed with the Cirque du Soleil show but it's a hot topic again:
http://www.eonline.com/news/marc_malkin/move_over_tupacmdashmichael_jackson/309702As the Tupac appearance was discussed, there were other virtual concerts with Sinatra, Elvis (as mentioned), and other artists mentioned as "virtual concerts" that were already staged and which fans already paid to see, so if it's not the fact that the hologram is 3-D and more lifelike if that's possible, it's not necessarily a new concept. And the Jacksons had this in mind two years ago as the article says.
Remember Natalie Cole's televised duet with her father's image projected on a screen? This just seems to be the next step. I guess the Cole performance sparked a lot of debate too, if I remember.