If the increased violence since February 2006 was from Sunnis killing more Shias, that would indeed be an escalation. But after taking it on the chin for decades, the recent increase comes largely from the Shias hitting back with their larger numbers:
The death rate has been increasing as Shia Arabs undertake more revenge attacks on Sunni Arabs. The government does not release data on the religion of victims, but reports from Iraqis indicates that over the last year, an increasing portion of the deaths are Sunni. Two thirds or more of the dead now appear to be Sunni Arabs.
Given that Shias outnumber Sunni Arabs by a 3:1 or 4:1 ratio, Shias are still being relatively restrained if they are killing Sunnis at only twice the rate they are being killed by Sunnis.
It would be kind of like saying that the rate of violence between Americans and Nazis increased dramatically after June 6, 1944 and concluding from that fact that the war was being lost.
So while this isn't pretty, and it hinders Sunni motivation to join the government, it is not fair to ignore the nuance of the situation to proclaim defeat.
And really, by inspiring Sunnis to flee, the Sunni terrorists are essentially draining the sea in which those Sunni terrorist fish must swim to be effective. The Sunnis will end up dead or defeated and the Shias really won't care which as long as Sunni terrorism is ended.