Steam banning MW2 CD Keys bought from ‘illegitimate sellers’
If you've bought a MW2 CD key lately from a so called 'online cd key store', such as G2Play and CdKeyHouse, you'll probably end up getting your game revoked from your Steam account soon.
According to official Valve employees this is due to the fact you've bought the key from an "illegitimate seller".

http://forums.steampowered.com/forums/showpost.php?p=12092791&postcount=20
However, rumors are going around that is a yet another money issue and that Valve/Activision is banning all cd keys not purchased from your own region. And since most cd keys from online cd key stores are bought in Asia, you'll end up with not being able to play MW2 anymore.
UPDATE: Current rumors state that this is incorrect. A lot of people which bought their retail MW2 in Russian via a webshop, didn't get their key banned.
In contrary what some people think, the keys are not stolen. They are sold in a market where it is priced cheaper because people don't earn as much (Asia, Russia) as in your own country. Therefore they are not for sale in our market.
Online CD Key stores often buy cd keys in Asia for low bulk rate prices to be able to sell them for profit online.
Also, this has nothing to do with the "trojan issue" which has by the way been confirmed as an false positive, in other words a not a real virus, by Avira.
Official Valve employees suggests affected players to "contact the seller to request a refund" and "in the future to only buy unopened retail boxed copies from legitimate retailers or purchase via legitimate digital download services such as Steam and Direct2Drive".
I guess this is the end for those trying to purchase a cheap copy of MW2 for PC 'legally'.
Sources:
http://forums.steampowered.com/forums/showthread.php?t=1031727
http://forums.steampowered.com/forums/showthread.php?t=1031758
Thanks to Frank for reporting this to us by email !
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