Right now, the only ones fighting the ISIS are the rebels.
Which is not so surprising that the dictator is not going after them, but what is surprising is that the ISIS is also not going after Assad.
http://www.nytimes.com/2014/01/13/world/middleeast/syria-rebels-turn-against-most-radical-group-tied-to-al-qaeda.html?_r=0
While both the ISIS and the dictator's forces share a soft spot for killing civilians, they cannot stay at an informal truce forever. The regime waited until the ISIS was clear before bombing rebel positions during their infighting, and they refrained from attacking the ISIS directly, but I definitely do not think that this is because they are actually aligned with the ISIS. They are both just having a lull in fighting each other to finish off the moderates, which are the ultimate enemies for both of them.
Assad has been going on a mass killing spree killing spree, blowing up markets, bread lines, hospitals, schools, neighborhoods, starving out cities, and a MASSIVE campaign of abduction/torture/killings of civilians. It's a basic dictatorship rule by terrorism, a tactic taught to him by his father and his father's institutions of terror.
When the civil war broke out, the greatest threat to Assad was the moderates who could finally establish a democracy, and create a viable alternative to his bloody dictatorship. By assisting the most extremist of rebel groups, Assad is both helping to destroy the moderate rebels who are his greatest threat, as well as showing the Syrian people that the alternative (the ISIS) are also torturers and murderers, so they might as well as go with the torturer and murderer that they know (Assad).
The ISIS for their part believes that the breathing room they are being given by the dictatorship will allow them to consolidate power and resist any enemies either the rebels or the government as they have been able to do with Malaki.
The ISIS blames the Saudis and the US for the backlash against them, but while the Saudis may hold sway. Al Nursa would not take orders from the US. The ISIS forced the rebels to respond proabably sooner than they would have liked because of their attacks on rebel bases, mass detentions, mass killings of prisoners like
Dr. Hussein al-Suleiman Ahrar al-Sham, and the public backlash at their harsh rule.
Assad is definitely looking to capitalize on this by moving on strategic resupply areas in Aleppo, but Aleppo is now relatively clear of ISIS. The real fight is in the North. Although their inability to refresh the forces in Aleppo could make a major difference. On the other hand, if the rebels can hold out, they could clear a major obstacle for receiving funding, especially since the removal of the ISIS was a major component of the Saudi plan to invest
"several billion dollars" into the development of a 40,000 to 50,000 strong Syrian National Army.