I found this backport patch for kopete 0.12.7 / 3.5.10 at http://forum.kde.org/viewtopic.php?f=18&t=78791&start=30 / http://launchpadlibrarian.net/33036596/kopete_yahoo_update_login.patch that resolves the recent yahoo login issue. Since I didn't find any gentoo related solution yet, I decided to try it myself. So I downloaded that patch, modified the latest kopete 3.5.10 ebuild and emerged it. Now my kopete can log in to yahoo again. :-) (But I haven't had the opportunity yet, to try out if chatting works also.) Thought I'd share the stuff and hope it works for other kopete 3.5.10 users too :-)
Created attachment 206873 [details] Modified ebuild, includes the yahoo login patch
Created attachment 206875 [details, diff] Diff against portage tree's kopete-3.5.10-r4.ebuild
Created attachment 206876 [details, diff] The yahoo login patch. Put it in files directory http://launchpadlibrarian.net/33036596/kopete_yahoo_update_login.patch
KDE 3.5.10 has been masked for removal, this won't be fixed...