In my experience the answer you get depends on who you talk to.
All the front country and telephone staff I've dealt with were absolutely insistent that the ursacks are forbidden and you must use a can. I've had a few debates with them about this over the past few years.
However, after a few emails to and fro earlier this summer I eventually received confirmation from the Inyo wilderness manager that they were indeed permitted, albeit only in Inyo. I asked specifically about the ursack hybrid. Whether the non-hybrid model is acceptable is another matter.
I've also been explicitly permitted to travel through the canister-required zone without food in a can or ursack, on the basis that I was running and would definitely make it to the next bear box. Even the normally-inflexible front country staff agreed this, and the backcountry ranger who met me also confirmed it.
