Chelsea FC has been banned from signing any new players until January 2011 (which amounts to 2 transfer windows) after FIFA's Dispute Resolution Chamber found that the club induced a teenage player, Gael Kakuta, to break his contract with his French team, Lens.
Both Chelsea and Kakuta were also ordered to pay Lens 780,000 euros in compensation. Meanwhile Kakuta has himself been banned from playing for 4 months.
Chelsea have indicated that they intend to mount a vigorous appeal against the decision. Any appeal is likely to be found on the argument that the severity of the penalty is effectively an unlawful restraint of trade.
A similar situation arose in 2005 when AS Roma were banned from signing players for 2 transfer windows for having induced a French teenager (Philippe Mexes) to break his contract with Auxerre. That ban was ultimately reduced to 1 transfer window following a successful appeal to the Court of Arbitration for Sport.