I just upgraded 5 OpenACS 5.0.4 servers to 5.1.
Two presented messages regarding ad_context_bar or something. I thought I would have found it in the log. Restarting AOLServer manually after the kernel upgrade allowed it to restart properly at which point I was able to upgrade the rest of the packages.
Somewhere along the line, something changed in one of the templates, and alas, I don't have the javascript in 2 of the 5 sites. I'm sure its just a missing piece of code in the .adp for those two sites. It prevents upgrading packages through the main upgrade procedure unless you click on the package name one by one.
Other than that, the upgrades went smoothly. One comment from a client was that the site appeared to be slightly faster.
I must say, congratulations! Since I started using OpenACS, it has improved considerably and just keeps getting better. Great work everyone!