Sounds like Linksys have finally admitted (to some people at least) that their new-ish WAG354G modem/router doesn’t work with the PS2. I’ve been trying to tell them that for months, but this post at linksysinfo.org is about the most informative (and least encouraging) I’ve seen about it so far.
I’m not completely hosed as I can still swap out the Linksys for my old ZyXEL modem (no router) and run a cable downstairs from that straight to the PS2, but that’s not quite the wireless utopia I was hoping for.