i have not encountered any major glitches in upgrading four blogs to wordpress 2.6 but there is this tiny thing that annoyed me a little. after upgrading wasn’t able to log in to the dashboard. it just keeps saying “please log in again.” even though i’ve tried logging in several times already.
the solution i found was that you have to log out of wordpress first. go to the home page and, if you still have your meta section in the sidebar, click on log out. and then log in again with your usual username/password combination.
i think this has something to do with wordpress 2.6 using three cookies instead of the usual one. doing the steps above sort of cleared the previous version’s cookie replacing it with a newer version. i think. 
You’re right about the cookie problem, I think. I just updated to 2.6 myself and had the same issue, and deleting my cookies allowed me to log in again.
THANK YOU!!
I have been pulling my hair out the last hour – re-uploaded WP about 4 times, edited my P/W in mysql, tried the emergency password hack, and this was the answer. What a pain!
you’re welcome. glad to be of help.
Thanks, a couple days ago I went to the trouble reinstalling the update severals times and then just started over.
When it happened this time I decided to find an answer and this one worked for me.
Thanks for taking the time post this, it really helped.