I think Drupal is a good choice. From what I learned about it, it has compact core and lots of modules to add functionality. It was also featured on this year’s GSoC (and was popular among the GSoC students), so it appears solid and modern and developing. The biggest problem I had with Drupal was making it easily-multilingual. This is definitely achievable, but isn’t the core functionality of Drupal (or wasn’t? I’m half a year behind the latest version of Drupal).

User sharing among different PHP-based web apps is a relatively easy task, and often requires only a bit of custom programming. However, the effort and time required depend strongly both on the exact level of integration you want to achieve, and on the differences in user privileges system. The easiest thing to do is usually enabling users to cross-login at different CMS-like systems (e.g. WP and Drupal) after registering in one of them. However, such custom solutions might not be easy to maintain with future upgrades, as “linking users” requires some mapping of the user privileges…

There are also such controversial things as OpenID and the like, and if both CMSs support it – it’s the easy and compatible way to go. Though I doubt it’s a secure way to go

BTW, if you are looking for WP-compatible CMS, why not just use some customized WP install? Do you lack some specific features, which are not available as WP plugins? I’m asking because the easiest way is always using less tools, if possible

I have decided to give Drupal a try – If you ever come accross a PHP network that integrates well (shares users) with WordPress & bbPress please let me know.

Best Regards,
Jeremy