Just to explicate this topical but rather lame pun, which has been appearing on email signatures, T-Shirts etc.:
rm: This is the Unix command which "removes directory entries", or deletes a file, in other words.
-rf: This indicates that you wish to use the -r and -f options of the rm command. These options work as follows:
-r: "Attempt to remove the file hierarchy rooted in each file argument." That is, if the file specified is a directory, delete all subdirectories and files encountered underneath it.
/bin/laden: This is the part with the lame pun. /bin is a directory on Unix systems and workalikes which contains important system commands and the like (e.g. the abovementioned "rm", the much-loved "ls", the enigmatic "cat" etc. etc.) Well, imagine if there was a command called "laden"? It might live at "/bin/laden"! Ho ho.
-f: "Attempt to remove the files without prompting for confirmation, regardless of the file's permissions." In other words, don't bother the user with warnings that they may be doing something stupid.
In summary, then, this impenetrable slogan is the geek's way of calling for the removal (rm) of Osama bin Laden (/bin/laden) and all his subordinates (-r), with extreme prejudice (-f). Once the skilled cryptanalysts of bin Laden's al-Qaeda network have deciphered this message, they will undoubtedly be stricken with a Great Fear, for they will know that they face not only the military might of the United States and NATO, but also the ire of 7th-level sysadmins with mad Unix skillz.