After upgrading to WP 2.6, I have this CSS error each time I select the U icon in a post to create or edit a UMapper map; I am using the UMapper plugin 1.1.76:
Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'media_admin_css' was given in /home/[...]/public_html/wordpress/wp-includes/plugin.php on line 311
The other plugins I'm using are AddThis Social Bookmarking Widget 1.0, Akismet 2.1.6, Google Adsense for Feeds 1.1, reCAPTCHA 2.5, Smart Ads 1.2, WordPress.com Stats 1.2.2 and WP PicLens 1.0.5.
At the time I'm posting this, I also noticed the service is extremely sluggish, so I can't even bring up a map to edit with the plugin and they take an extremely long time to load in my blog (example: http://www.davidmattison.ca/wordpress/?p=2365).
Other than that, the plugin works fantastically well and I'm very pleased with its ease of use, especially the new geo and Wikipedia search capabilities.