I'd been having a lot of trouble with circles all printing on my Printrbot Plus with corners, the term 'lemon' and 'football' circles are used by folks with similar problems. This appeared to be an X backlash problem since it seemed that the system was lagging when adding X vector to the Y motion.
My X-Axis belt was really tight, so it wasn't obvious backlash. However, based on the PB+ assembly instructions step 142 (sorry, system won't let me post an offsite link) , the bolt connecting 220 to 232 had a nut on the OUTSIDE. The belt was actually pulling the motor mounting frame out of square. It was positively trapezoidal! This was causing the gear on the motor shaft to rub against the x-axis main frame.
I took it apart and put the nut on the INSIDE of 232 (see picture below). This serves as a more effective stand-off than the wooden peg and let me true-up the mount. The belt tension holds the motor against the frame, but I'll add another nut on the OUTSIDE for insurance.
Now the system is printing GREAT!! You can see the difference in the (mr. blurrycam) calibration circle before and after: