Well, I finally got around to upgrading WordPress again. I made a fairly prodigious leap from 2.5(.somethingsomething) to 2.8 in one step, with pleasantly few hiccups.
In fact, the only significant hiccup I faced prevented me from logging in at all (fairly significant), giving a charming error: “You do not have sufficient permissions to access this page.”
This happened after I went to update my wp-config.php file to incorporate a circa-2.6 innovation, Security Keys. AUTH_KEY, SECURE_AUTH_KEY, LOGGED_IN_KEY and NONCE_KEY added to my wp-config, I attempted to login to the admin area again (it auto-logged me out, fair enough) to be greeted with the aforementioned non-negotiable error!
Turns out it was a pretty stupid problem I had… If you get this, check you have inserted the key definitions before the “Stop editing” comment in the document (I think the comment is different in later versions — my wp-config is seriously ancient, I’ve been upgrading since 0.73 and would only have started from scratch if at some past point it was required!)
Once I did this the errors went away entirely and I was able to login.

Thanks for posting this so I could find it on Google! You saved me hours of searching for a solution to this problem.
Sure nuff, moving these above the /*stop editing*/ and everything is back to normal.
After pouring over the database, plugins, file perms, and all manner of php hacking. this turned out to be the issue for me as well.
I think that some sites that don’t have as much version history as ours must not be as sensitive to the placement of these keys in wp-config.php.
Woot, and Thanks!!