Those of you who bought Modern Warfare 2 on Steam will not be able to play until Thursday.
Meanwhile, those buying a PC copy of Modern Warfare 2 from shops today can play immediately.
Both versions use Steam DRM procedures, but Valve has not explained why the Steam-bought copy requires an extra two-day wait to play.
Modern Warfare 2 was released around the world last night. The UK launch took place in London's Leicester Square, overseen by celebrities such as Vernon Kaye and one of the Black Eyed Peas.
Our Modern Warfare 2 review was published moments ago.