It's more than likely gonna be Floor. She's on vacation with the band right now, she'll be singing on the live DVD they're recording next month and she's also gonna be with them for their shows early next year and a few of the festival dates they have set for next summer.
It seems like she's becoming more than just a temporary vocalist.
6/5 - Guns N Roses
6/14 - Alice Cooper
7/6 - RUSH