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.
-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.
/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.
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. | 
