I'm glad that I'm not alone with the admin panel comment editing problem after upgrading to the earliest version of WP 2.8. As I hadn't been posting new entries in my watch blog since last April, I didn't notice any problems until very recently.
I did not realize that DB Cache was incompatible with version 2.8 upwards, thinking it was a quirk of WP 2.8. Currently I'm running WP 2.8.4 and after seeing that the comment editing and tagging problems persistent, I Googled for an answer and found a post stating that DB Cache was the culprit.
I disabled DB Cache and tried to edit one of my readers' comments. It worked flawlessly. When I re-enabled DB Cache, the symptoms returned. I later found out that the author of DB Cache has yet to update this plugin to work with WP 2.8.x.
IMO, my blog appears to perform faster with DB Cache enabled (along with WP Super-Cache), therefore while waiting for DB Cache to be updated for compatibility with WP 2.8 all I have to do is to temporarily disable it when I have to edit comments and turn it back on when I'm done.
I know it's not an elegant workaround but I'm already addicted to DB Cache until I find something equivalent or better. :-)
On an unrelated note, I also discovered that the automatic upgrade of WP core files within the admin panel usually fails unless I manually disable all active plugins. While the instructions tell you to back up your databases before upgrading (of course I do backup my WP database before performing the auto upgrade), WP unfortunately doesn't tell you to disable your plugins first.
I assumed that the upgrade job will take care of that for you (for example, upgrading an individual plugin within the admin panel will automatically disable the current plugin first), but it looks like WP doesn't disable all of your plugins.
You have to remember to disable all active plugins first before performing the automatic WP version upgrade and go through the list of plugins that require manual enabling after upgrading to the latest WP version. A good example is the WP Super Cache plugin, which always defaults to "OFF" whenever it is disabled albeit temporarily.