Daemonology provides an upgrade script to binary upgrade FreeBSD 6-RELEASE to FreeBSD-6.1. The procedure requires that one simply download the tarball and run upgrade.sh. I used this approach on ziggy with good results:

FreeBSD ziggy.multifx.net 6.1-RELEASE FreeBSD 6.1-RELEASE #0: 
Sun May  7 04:32:43 UTC 2006     
root@opus.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  i386

and recommend this procedure to anyone who wants to upgrade their system.