Is there any clean way to have the Drupal login form create a session for phpBB also? I've made my peace with having two DB's and two sets of roles, permissions, etc. But I'd like to have just one login.

Is there a simple way to have the Drupal login throw a session cookie for phpBB with the same parameters as the drupal one?
If what you want is a clean way to integrate phpBB with Drupal, I think phpBB Forum Integration is want you need.

This module will not only "embed" phpBB within a Drupal site (and synchronize account info, such as user names, roles, passwords, emails, signatures, and avatars, with only one login), but it also let phpBB topics be created from Drupal nodes.

The phpBB forum can be displayed in a full window, in a frame inside Drupal, or in a Drupal page.