Rounding is a major concern on just about anything that does a significant amount of calculation.. It wouldn't be surprising at all if mtgox got that bit wrong... compared to some of the other stuff I wouldn't even consider that an issue right now.  OTOH from the initial 1BTC example it looks right (0.0065 rounds to 0.006 using bankers rounding).

btw. you never compare floats for equality.. you compare within a tolerance, or don't use float.  I'm not sure why mtgox would need to be comparing floats anyway.. mostly they're showing the results of simple calculations.

That 0.2222 error.. I've made that kind of mistake myself by rounding too early.. it happens.  It'll probably be fixed by the code rewrite and if it isn't then's the time to point it out, not now.