I am getting that same message as well and using the same WRT54GX router as u are. funny thing is that it was working fine last night..I was able to sign in and download tekken 5. However I had some other demos lined up and it wouldnt download. Turned of the system and nothing, redid the network configuration and got this error all of a sudden.
I hooked up my ps3 and got on the net straight away no problem, downloaded demos e.t.c even bought game, the all of a sudden it won't log on, i occasionly get an all clear internet test but keep getting the dns problem.
Since this i havn't managed to log on for more than 10 secs before being logged out.
one thing i noticed is when it dettected my ip, sometimes it gave me different ones that worked but ussualy 192.168.100.10
I am running straight into a modem and have no connection to pc at all, is this ok cos it worked for 2 days.