I have just had a conversation with a guy from Michelin and he has told me the mark is a antistatic strip see email:
In summary no need to worry
"Thank you for your e-mail.
From what you describe they sound like the anti-static strips that are in the
tyre tread.
You can continue to run the bike at normal speeds and these strips will
'disappear' within a few hundred miles .They have no detrimental effects on the
performance of the tyres.
All Pilot Power/Sport/Road tyres have this strip but only a few riders notice
it!"
I have attached a 'simple' diagram as explanation.
Pilot Power
A circumferential groove approx. 2 to 3mm wide and 3 to 4mm deep
Two Wheel Department,
Michelin Tyre Public Limited Company
30-Jun-2005, 18:53 
Threaded Mode