We finished the upgrades around 8pm (PST). It was surprisingly easy to upgrade from RHEL3 to RHEL5. The only thing we've run into so far were some of our custom built things we had compiled with the system OpenSSL that needed recompiling.
Most services are up again (and have been for some hours). We're upgrading our monitoring and system management tools and we're still tweaking and configuring, so do let us know if you see anything broken.