I upgraded RT3 from 3.6.5-3 to 3.6.7-1 and lost the ability (even as superuser) to assign tickets to anybody. When I pull up the "Basics" screen for a ticket, the "Owner" drop-down only lists "Nobody" (none of my users are in there). I rolled back to 3.6.5-3 and it works again.
It looks like a newer version of DBIx::SearchBuilder is required. I grabbed 1.54-2 from rawhide, rebuilt it for RHEL 5, and RT3 is happy again. See also: http://lists.bestpractical.com/pipermail/rt-users/2008-March/050830.html I'll file an update request for DBIx::SearchBuilder I guess.
perl-DBIx-Search builder was already updated in EPEL 5 on my request, to fix this issue. I've since then pushed another rt3 release which enforce the requirement on both perl-DBIx-SearchBuilder >= 1.53 and perl-Devel-StackTrace >= 1.19 (security fix). This new rt3 release is available in EPEL5-testing. It's not flagged for the next stable push, but I will ask Thorsten to add it to the push.