I used to run AdCritic.com on a single server installation (dual 800mhz proc, 1gb ram) running both Apache + Mod_PHP and MySQL. I was doing close to 1 million page views per day, with the capacity to handle 1.5-2 million. I could have added an additional server to increase my capacity to 3 million pages. I could have moved to 3 machines, 2 front ends and a mysql DB on the back end to get up to 4-5 million page views.
Every page view was running sessions, and over 50% of the page views were to logged-in users. Every page was generated from the DB. Sessions were stored in the DB.
I didn't use the PHP accelerator -- I wrote well-crafted SQL statements, optimized my DB, used persistent connections to the DB, optimized code and continued review of all code.
You don't need the PHP accelerator to make PHP scalable.
BTW, this was between 2000 and 2001 using PHP3 and the early versions of PHP4. I'm sure that using PHP5 the site would be able to handle more page views with the same hardware.