Find the detailed instructions here: http://erhnam.demon.nl/asus-wlan-hack/ . Alhough the webpage is dedicated to WL-300, I can confirm the Telnet daemon activation works fine on the WL-500b (1.6.4.6, 1.6.5.3) as well
Update: The FTP root trick does not work on new 1.7.5.6 firmware anymore...