This NeoGAF thread covers the recent discovery of a new flag on Steam games called "AllowCrossRegionTradingAndGifting," which from the name appears to allow region-locking for Steam trades and gifts. Word is this is present in the just-released METAL GEAR RISING: REVENGEANCE where the flag is set to "no." According to the thread, this seems to currently only apply to South America and Eastern Europe and that it does not affect retail game keys. Thanks PCGamesN.