Upgrade drupal using drush: Failed to check available update I am trying to upgrade Drupal from 7.14 to 7.18 using drush up command.

Nothing gets updated and I see a bunch of failed warnings:

Failed to check available update data for CCK.
Failed to check available update data for CKEditor.  
...
Failed to get available update data for 51 projects.


I can however download modules using drush dl module_name
All commands are run as root.
As tenken suggested, you might try adding -d for debug to your drush command or -v for verbose.

Beyond that, I suggest trying to set $conf['update_max_fetch_time'] to a higher value (I usually set it to 360). It sounds like you may be running into an issue where fetching the update information is exceeding the timeout.