Posse comitatus is already decided, the "Incident Of National Significance" trumps it, IIRC.
Well tell that to the military spokesman at the FEMA news conference 2 hours ago, who responded to questions of why isn't there more nat'l guard being deployed into the city with lots of rambling about how the military is very restricted in what they can do, can only support, not supplant, civilian law enforcement, must be called in by the governor, etc. Sounded just like the earlier excuse-making of the governor. Ridiculous that such was still going on.
That's all pomp and circumstance, FWIW. Federal force is always available, with the extent of reaction being based on extent of need, with no violation of Posse comitatus. Go through the right formal motions, and US military can be on any street, with the only limitation being logistics.