Wordpress 2.8.5 and 2.8.6 as packaged with Ubuntu are broken. Using the admin pages gives the error:
Warning: constant() [function.constant]: Couldn't find constant WP_CORE_UPDATE in /usr/share/wordpress/wp-admin/includes/update.php on line 33

A bug report addressing* this problem can be found here.

I have a simple/ghetto workaround for this issue. Open:
/usr/share/wordpress/wp-admin/includes/update.php
and comment out the two lines:
if ( !constant('WP_CORE_UPDATE') )
return false;

!! You will need to edit this file with root permissions. If you are running a full desktop, you can do:
sudo gedit /usr/share/wordpress/wp-admin/includes/update.php &

If you are running a headless server as I am, you can open the file for editing using vim, nano, emacs, etc. ie:
sudo nano /usr/share/wordpress/wp-admin/includes/update.php

The error will no longer appear, and the blog will continue to function 100%. There are absolutely better ways to deal with this issue, but that is up for the Ubuntu package maintainers to .. maintain.

The following screen-shot shows exactly where to add comment notation:

* “addressing” might be too strong of a word considering the progress being made on this issue. Two versions of wordpress to go through the package maintainers and it’s still broken? In fact, the report is still considered “New”.

UPDATE 12-24-2009
Turns out the Wordpress 2.9-1 update still has this issue…