A short story with a happy ending: my Asus WL-500g locked up and it wasn’t starting, even after I hard-reset it (removed the power and plugged it back in). All the LEDs were constantly on (normally, when you plug in the power, they should light up for a second or so and the turn off). After some searching I found a page on FixYa which pointed me to the following forum thread: Dead or brick ? Lan and WAN 1-4 leds on steady. There were a couple of stories here with identical symptoms, so I decided to give the solution a try.
The tricky part was to find a source which could deliver 2.5A at 5V (most of the ones I found peaked at 1.5A). Finally I found this one locally and it worked like a charm! (It seems the trick is to search for “switch power source”). Also, for extra safety, look carefully at the adapter when you get it to make sure that it conforms to the specification (5V DC / 2.5A) and if possible, use a multimeter to ensure that the polarization is correct (in my case it was + on the inside, – on the outside).
PS. I didn’t get to disassemble my old source yet, but I will post photos whenever I do so.