I very much doubt the first Broadwell that will be introduced will be compatible with the X99 platform as they will be 4-core/8-thread max variants. I haven't read about it for quite some time, but the first Broadwell processors should fit on an LGA1150 of some kind. I remember Broadwell's launch has been delay. What was originally supposed to be a November or December novelty might very well wait until February. I don't know for sure and I hope I'm wrong on this.
Haswell-E, that's another story. Those will have up to 8-core for consumer chips and I expect those to fit on an X99-based motherboard. We'll know for sure during the second week of September, when they'll start selling.
Broadwell-E won't arrive before September 2015 during the early days of IDF San Francisco, assuming Intel keeps introducing the -E consumer processors at the same time as the enterprise-grade -EP Xeon.