That started me worrying too but the latest post gives a solid explanation which I'm happy with.

Would be interested to hear from Madison though.... 

If lots of people have the "crack" and yet no-one has reported a failure it implies that it isn't a significant problem. Stats do come in handy I find.

Taken from the STW thread - 

 Hi everyone, Albert from Madison here.

Understandably some of you are concerned about what might appear to be cracks at the tips of your crankarms just below the pedal inserts.

This was brought to Shimano's attention some time ago and upon further investigation they were identified as forging marks (Stato's hit the nail on the head with his explaination). This is why so many of you will have discovered them upon further investigation.

Rest assured these marks do not in any way affect the performance or durability of the cranks/chainsets and to date no cranks have been returned to us with actual cracks at the end of the crank arm.

Thanks
Albert
(Madison)