Thanks for answers guys.
@enderandrew Yes that’s the problem for me – I am going to have some other large WordPress project which I’d like to be integrated with this new forum very closely. That’s why I think about bbPress.
@John Yes WordPress itself is very scalable to start with Mashable and with itself WP.COM and it’s really all about optimization, traffic handling, hardware, etc. But which one will need more working on all these options – professional forum such as IPB ro vBulletin or bbPress?