I love Mean Girls and I love Tina Fey and I love both the musicals I've seen, so this wonderful news item contains nothing but good tidings.
Although, I must admit it's not actually news yet, merely some good intentions. During an interview at the SAG awards, Fey mentioned her desire to make a Broadway Mean Girls musical happen and that she and her husband are currently working on the project. This echoes past interviews where she said pretty much the same thing, as well as future interviews where she once again takes the same position. You could probably throw some parallel universe interviews in there as well.
In other words, who knows when we (and by 'we' I mean people loaded enough to go see stuff on stage in big cities) will see this, or if we'll see this. Though it's not like money people would have a hard time seeing the value in a Mean Girls musical, both for its own status as a fondly remembered hit film and the success these big time movies turned musicals tend to find (I write this statement from a place of pure, undiluted ignorance, mind you; perhaps all these plays seem successful to me because I rarely hear about the non-Spider-Man failures).
Obviously no casting has happened yet, not that it matters. It's Broadway, so the cast would likely be filled with girls you've never heard of anyway (all from Niagra Falls). On that note, Lindsay Lohan jokes are not appreciated here because the diseases that hound her are no laughing matter. Except for whooping cough.