As we find out recently Online Key Store is sending out new sets of cd-keys for the game so that people who bought it from their store can easily re-activate their game once more on Steam.
Also G2Play is refunding (according to their response on Twitter):
http://twitter.com/G2PLAYSHOP/status/5916068001
So if you bought your key somewhere else and you’ve lost your game please do not hesitate and ask for a refund or a new key. And do not be afraid to use online key stores in the future – things like that happen but we cannot let Valve threat people with removing games and such.
Have fun playing!
Update:
Here’s a cool screenshot thanks to Rad86N:

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Modern Warfare 2: Get your game or money back!, 5.0 out of 5 based on 6 ratings
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I would expect no less from a legitimated business.
Very good!
I can confirm this, already got refunded on nov. 20th just hours after writing a email.
Im still waiting for my key or refund from g2play.net. 3 Days now…
my friend got his new key from OKS today..
Oh yes… But i’d prefer them refunding thos shitty FEAR2 keys
they sold us, and they fucked us ! Basterds…