Cubify made some changes to the cartridge.
I just got a new ABS gray cartridge (Source Graphics has gray and white ABS in stock).
Because of the known possible cartridge binding issues I did not want to take a chance of getting a print failure due to cartridge / filament binding, so I opened up the new cartridge to run it on an external holder, not knowing there were recently some improvements made to the cartridges.
The inner cardboard spool is now narrower to prevent / minimize binding. The filament is now almost wound up to the top on a new full spool since the spool is narrower. (But has the same amount of filament as before - close to 650g).
Since there is now a gap between the side of the cardboard spool and the cartridge case, you probably don't want to push filament back into the cartridge because it could go to the side of the spool and tangle.
Externally the new cartridge looks the same, so you can't tell the difference.
Not sure when the cartridge improvements got implemented, but I would assume any new cartridge you are getting now, has the improvements.
The old cardboard spools were ~ 46.5 mm wide.
Then new cardboard spools are ~ 40.5mm wide. The diameter is still the same, and the rest of the cartridge looks the same.
So from the outside, you can't tell a difference.
The date on the cardboard spool inside of the new cartridge is 23-05-13.
The Lot # of the new cartridge I got is: X12052413-1ABS-IGR (ABS Industrial Gray)
The date on one of the old (wider) cardboard spool inside is 3-5-13.
The Lot # on one of the older cartridges is: X12050313-1ABS-WHT (ABS White).
The date of the implementation of the cartridge improvement could be different for each color or type of Cartridge.
I put the new spool and cartridge back together and ran a 6 hour print with the new cartridge without issues. - Nice.
Anyway, hopefully the binding issues are solved.
October 2013 Update:
Well, I still have some winding and binding even with the new spool design, so if one binds once, I take the spool out of the cartridge and run externally.