there is no straight yes or no answer to this question.
You may want to read about it here:
http://www.bbc.co.uk/watchdog/consumer_ ... ikes.shtml
and here:
http://flightmole.com/forum/showthread.php?t=1251
Personally i think that over Greece is not the only flying option so this cannot be treated as extraordinary circumstance. But knowing CA i am 100% sure that they will refuse to pay. In any case, you can file a claim with an"agent". They work on a no win - no fee basis. This is one:
http://www.aeroclaim.co.uk/ClaimNow.aspx
Important is that you know that in case you did not use a ticket you are entitled to a refund AND if flight was cancelled you are entitled to compensation. In your case - you accepted the new flight, you are not entitled to refund, but you might be entitled to EUR 400 compensation depending whether strikes in another country may be considered being of "extraordinary nature". Good luck