I strongly believe that with that mod your website will be spidered by google. However you should also check the following forum-topic on the official phpbb-forums:
http://www.phpbb.com/phpBB/viewtopic...=647503#647503
It's the mod_rewrite, you can set this up in just a small time and then all your topics, messages, memberlist, etc... will be with the html-extension and very very spider-able by google. Google takes this like honey. However your server sould be mod_rewrite enabled and you should have access to your .htaccess-file . Check my website
www.luchtzak.be if you want to see the -html pages in action! It's a php-nuke but that's the same.
ciao,
Bart