From what I recall from when I did my install two months ago, the ipw2100 and ipw2200 drivers were not included on the 2004.3 livecd. When I installed Gentoo to my laptop, I had to plug directly in to the router, and I downloaded the drivers later.
You obviously have net access through some other means, so I'd try either running "emerge ipw2100" after you've chrooted during the install process, or downloading them from
http://ipw2100.sourceforge.net/. Be sure to read through
this page in the Gentoo wiki before you compile your new kernel for the first time.