I just got around to reading some of the followups to the original posts in this thread, and wanted to offer the following in terms of an update -
1) I think the links provided above for integration are very useful, and although I've considered using the phpBB cookie for single sign-on purposes, I've chosen to use xoops as the "core" framework for a site where i need more functionality than is possible using WP alone.
2) Many other products have either exising ports as xoops modules or have a reasonably well documented method for integrating, including mediawiki, phpBB, of course WP, and many others.
3) the last thing to consider, is how your individual sites will be used, for example on my personal site, i've opted to install different software in each /dir which is suited to each area's purpose/use. the / root of my site runss on WP, since I don't need that much functionality. the other area's use xoops, with WP, and other modules as needed, but I view each "area" as a separate site, thus install multiple copies of each package as needed, I've done this for a number of clients as well. Sometimes it's needed, sometimes it's not. That depends on what you are trying to accomplish specifically. I have two projects that have come up recently, which will each require a large amount of functionality, and so, will likely mimic what i've done at http://www.hradil.us - running the root as WP, then each area using the right platform for the section's purpose as opposed to using either xoops/Wp in the root and trying to hack everything together. The entire sites will still support a single sign on to some extent, depending on the specific content area.
If you look at wordpress.org for example, the "forums" area requires a separate login from the "codex" area, which is fine, and suits the sites functional needs.
same goes for the xoops main site, dev site and docs site.
I think most users are used to sites operating this way, and since a wordpress.org "member" may or may not be interested in access to other specific areas, i don't think folks mind too much having to sign up more than once.
having said all that, sure in a perfect world, a user could sign up for one of my sites one time, then have access to the areas they need/want with appropriate permissions for each, without needing to sign up more than once, i don't think that's practical at the moment, except as i mentioned, in the case of things like xoops "modules" or integration hacks.
another good example is bbpress which runs this forum, one would hope that eventually, the logins can be easily integrated with WP or bbpress becomes available as a WP "plugin".
on the opposite end, one of the site's i'm working on will have a developers area, and instead of the typical sign up confirmation email, each user will be admin approved, since they will have access to potentially harm the site itself, we need more control over who can/does sign up and who doesn't. so, using a single sign on for that site, doesn't make any sense.
in a nutshell, there are many options available for integration of various products/features, etc. each of us needs to pick what works for us and for each site's specific situation. as long as we're using products like WP which is easily customizable, and things like the others i've mentioned, just about anything we can dream up is possible.