I did disable all plugins. And just to be sure, I renamed the plugins folder and created a new, empty plugins folder. Problem persists. Spent a couple hours today on this problem, got nowhere.
I've reinstalled wordpress 3.0, and downloaded it separately and re-uploaded the wp-includes folder, which someone else suggested. I also switched to the default theme temporarily to remove that as a factor. All along I've been clearing the cache and trying again. I'd consider downgrading back to 2.x, but it's been a long time since I backed up my sql database, I've been backing up the blog all along but didn't realize I needed to backup that database too, so I'm hesitant to revert too far back.
I haven't checked the error logs, where would I find them?
One other thing I've noticed since upgrading to 3.0 which may or may not be related -- the updates section is notifying me that the theme "Pool" has an update available, and tells me I have version 1.0.7 installed which can be updated, but I'm not using that theme. Weird, but may not be relevant.
Thanks for your help so far. Btw, I'm still getting a 404 message accessing your link above.