From the other thread:
From FIFA:
"Players may only be registered during one of the two annual registration
periods fixed by the relevant association. As an exception to this
rule, a professional whose contract has expired prior to the end of a
registration period may be registered outside that registration period. Associations are authorised to register such professionals provided due consideration is given to the sporting integrity of the relevant competition. Where a contract has been terminated with just cause, FIFA may take provisional measures in order to avoid abuse, subject to article 22."
http://www.fifa.com/mm/document/affe..._en_122007.pdf
So unless Drogba was registered as a free agent before the transfer window was closed, he will not be eligible to sign. According to FIFA.