FYI for others who might encounter this.

I just did a yum update (C5.1) and after a very long download of packages I get a transaction test failure:

Transaction Check Error: file /usr/lib/libc-client.so.2007 from install of libc-client2007-2007b-1.el5 conflicts with file from package libc-client-2007-3

I just realized this is coming from a conflict with a custom build I made of that package so that I could install uw-imap-devel as a prereq for building an asterisk package.

It looks like I can just erase those two packages and the yum update should go through.