Apparently, al Qaeda (largely run by conservative Sunni Arabs and supported by wealthy Gulf Arabs) has refused to assist Ansar al Islam (largely run by Shia Arabs and supported by Iran). It's unclear exactly what the refusal means, as the information was picked up from various intelligence sources (intercepted messages, "heard on the street" and interrogations), not from the horses mouth. Individual Arab terrorists have been heading for Iraq, and there are several al Qaeda cells operating. But they are doing so in an increasingly hostile environment. Most members of Ansar al Islam are Kurds, and these are even easier to pick out down south around Baghdad and Basra. The fact that that Ansar is seen as a tool of the Iranians makes them even more unpopular with Iraqis. Meanwhile, the suicide bombings continue, or at least the attempts at it. The bombing teams have been increasingly frustrated by security measures, and forced to detonate their bombs among Iraqi civilians, or abandon attempts, because of this. The dead Iraqis have made Iraqi civilians more willing to turn in terrorists (and collect rewards for doing so.) 

Al Qaeda and Ansar al Islam are both upset that they have not had more success in Iraq. But both groups have not given up. While the current split merely exposes long standing hostility between the two branches of Islam, it also shows that there is a unity of sorts against their common enemy; non-Moslems. As long as Moslems (mainly Arabs)  are willing to volunteer for the cause of Islamic militancy, the violence will continue. It's not fashionable to report this as a religious war, but to the terrorists it is. Just because one side in this conflict downplays the religious angle, won't make it go away.