I am following this thread carefully. I have a dashboard issue, very similar, but not exactly the same. I don't see the dashboard at all for my main site.
I run 14 WordPress sites. All are on WP 2.9.0 at present, except the one giving me trouble. That one is "upgraded" to WP 2.9.1. I completed the upgrade (using automatic method, after backups), then went into the dashboard to do maintenance.
No dice.
I'm using Firefox, latest version I know of (3.5.7; automatically updated).
First, I logged into my site. That worked well, and recognized me properly. Then I clicked on the Dashboard link in my sidebar login space.
I saw the dashboard start up, but in less than a second it disappeared. The tab went from showing "Dashboard" to showing the raw URL. That tab continued to show the action as incomplete, even after the bottom notification banner said it was Done. The displayed URL is correct.
I tried both the admin user, and my own user w/ admin status. Again, no dice. I hit the back button on the browser, and it appears to be stuck on the blank page. If I hit the back button twice quickly, it goes all the way back to my blog site. (www.chileunderground.com)
I downloaded a new package of WP 2.9.1, latest stable code. I replaced the wp-admin files and folder, leaving the rest of the install intact. I didn't attempt any edits of the database.
I next tried Internet Explorer 8, and everything is fine over there. Apparently, Firefox 3.5.7 and WP 2.9.1 don't play together nicely. I'm trying to get Firefox 3.6, but it's slow downloading; I'll post on that experiment later.
All my WP 2.9.0 installations are fine. I won't update any of them until this issue is resolved.