I'm using PHP5.1.2 on Apache/1.3.33 Ben-SSL/1.55 in Debian r3.1. My eAccelerator when I noticed the issue was 0.9.5-Beta1, though have also tried SVN trunk checked out on 2006-03-22, 20:52 UTC.
When accessing phpBB's installer, it dies with the following error in the Apache error log:
[Wed Mar 22 21:13:11 2006] [error] PHP Fatal error: Class 'install_main' not found in ./install/index.php(234) : eval()'d code on line 1
If eAccelerator is disabled (say via .htaccess with: php_flag eaccelerator.enable 0 ... then phpBB's installer will run normally.
phpBB's CVS is here: http://cvs.sourceforge.net/viewcvs.py/phpbb/phpBB2/
Said revision of file, that's apparently dieing: http://cvs.sourceforge.net/viewcvs.py/phpbb/phpBB2/install/index.php?rev=1.4&view=auto