Update: The packaged version of Zend Framework is currently quite out of date (5 versions behind the current release at the time of writing), so I would recommend grabbing the latest release from http://framework.zend.com/ instead of installing using the method below.
With the release of Hardy, the Ubuntu repositories now include a package for the Zend Framework, so you can have just one copy of the library on your server that is automatically updated. To install and use this:
sudo apt-get install zend-framework
then add it to the include_path for your app, in a .htaccess file:
php_value include_path '.:/usr/share/php/libzend-framework-php'
you can then require in Zend Framework classes as you need them, or use the Zend autoloader to pull them in automatically when instantiated.