I've had PEAR act up on 3 different Natty installs, generally it's shirtiness manifests itself when you go
$ sudo pear upgrade-all > PHP Fatal error: Call to undefined method PEAR::raiseErro() in /usr/share/php/PEAR/REST.php on line 165 PHP Stack trace: ...
You can fix this nuisance:
$ sudo mkdir -p /tmp/pear/cache $ sudo pear upgrade-all
Hope this helps save someone some time!