After upgrading to 2.6 all seems to be fine except for the WordPress.com stats plugin and the Plugins menu.
The WP.com stats gives me this error message:
Your blog was unable to connect to WordPress.com. Please ask your host for help. (transport error - could not open socket: 0 )
I read about it in the forums, and contacted my hosting provider.
They says that the ports are open, so I must be able to use this without problem... and in fact, I was using it fine just before upgrading...
Also the Plugins page took a lot of time to load, I supect that is due the same problem because of the checking updated plugins, so it waits to retrieve the info...