actually, think its the other way round - Assads' faction (and it is a faction, they probably control about 30% of Syria - people who only control 30% of the territory they lay claim to aren't its government) appears to be the one anvil IS can't crack, i don't believe that for a second he's about to go on Camerons Christmas card list, but i doubt that attacking Assad is something the west will do before IS is dead, buried, dug-up, hung, drawn and quatered and had its bits put on spikes.
a diplomatic solution against IS, one where Assad retires to a sun-drenched paradise and a much wider Syrian coalition both ends the civil war and takes on IS is a much more attractive proposition for the west, but Libya and the experience of working with the various Syrian opposition groups is likely to dull any enthusiasm, and they'll decide to go for a 'meh' solution thats in place rather than a 'good' solution built from fairydust.