I am a newbie to wp.
I applied a theme written for the latest version of wp to an older version of wp. so i got the undefined function wp_get_theme() error.
This then blocked me from my access wp as admin
I google the function name to learn this is a new function only in the latest version.
So this is how i was able to upgrade without admin access.
Using ftp, I copied all the files from the default theme twenty ten
into my theme that was created only for the latest version of wp.
this then allowed me to then log in as admin and upgrade by clicking the upgrade button. After the easy upgrade.
I then restored the original wp-foundation theme files back to the folder and then logged in as admin and switched back my new theme that works great with the latest wp version.
hope this helps someone.