I have a WRT54GL (only real difference is Linux instead of VxWorks, and some better hardware)..

Just tried logging into with the WRONG username (tried blank and random characters), and right password, failed.

"They simply don't use usernames." <- Bullshit right there.

Tried logging in with just admin, failure.

"It does, it just logs in with the username admin no matter what." <- Sure it does.

Tried it again, same password, but this time the correct username, and correct password, works.

"Well then let's call it root. I'll concede to that since most of them (barring only the V.5's) run a very stripped down linux kernel."

As far as I know, the WRT54G models do not run on Linux. They run on VxWorks. Read this: http://www.wifinetnews.com/archives/006101.html