Joomla integration

I have a Drupal site for a committee, and my Society has a new Joomla site. I don't expect to have by Drupal site absorbed into the Joomla site, but the question will come up. Assuming that can't be done, how easily can I link Joomla pages to Drupal content (assuming common usernames and passwords)?

The same applies for me.

Here's my situation: There's someone I would like to help very much with building, or relaunching, a community website. Would have loved to do this with Drupal (and still would love to convince him and the other guys). Currently there are just some static pages and a phpBB, and this board and the users should be integrated in some way.
But the guy who is doing the project management, assigning tasks and so on, is a Joomla addict. Well, all his projects (Joomla or PHPNuke) look rather poor.
I, personally, would love to use the advantages of CCK and Views for this community, as it could be very cool for building a knowledge base / database - the community is about a tv series, site owner is one of the actors of that series.
There will probably be a lot of editors who are not very familiar with web publishing. So I think it would be very cool for them to have their edit tab or all their other permitted admin or editing actions just inline, on the page just where they are. Logged in some days ago on their Beta site, it was the first time I saw the backend of Joomla (as I love Drupal and there was no reason for me so far to install Joomla). It was horrifying to see that, I think a customized Drupal, perhaps with a dashboard could be much more intuitive to use for the editors.
And of course there are the field-based permissions which could be cool for editing, the flexibility of views output (example: fetched a certain feed from Youtube and made Views show just one random video of this feed on the front page). And OG and ... well, I think you all know the big Drupal advantages.

To make it short, the development is not in a stage yet that changing to Drupal is impossible but rather unlikely. I thought I had some good arguments but JFusion seems to be much more mighty für integrating phpBB than the Drupal bridge for that.

But the "boss" (the actor) has some other sites / domains, also related to that series and it is indeed possible that I'll get a project of my own - but then it should be part of a "webring", as he calls it. So a single sign on with the - then existing - Joomla project would be a big advantage. I don't know if LDAP (don't even know how it works exactly) or OpenID are alternatives.
I also have to say that I used FBconnect successfully on a prototype I made (and I'm going to show these guys) - many of the users also have a Facebook account so that would solve parts of the problem.

I'm also surprised that there seems to be nothing comparable to JFusion for Drupal yet - a seamless integration for all the popular message boards, and not just a bridge. Synchronizing the user DBs would be cool, single sign on and staying logged in would be even better (don't know it that is possible over several Domains, for Cookie reasons).

What I want to say is: although it doesn't seem logical because Drupal is a CMS and Joomla is a competing CMS, there are also some use cases where integration is needed - have people with Joomla skills and some with Drupal skills working on several sites which should be connected.
Or, like vcasinha, having some old sites based on one system, switching to another system for new sites, and wanting to connect them.
My question is also: are there some major difficulties in connecting the logins?

I also thought of just migrating the data from the old board to a Drupal system (in case they would not use Joomla) - the old board has no advanced features so the look&feel and features of Advanced Forum should be sufficient especially as it would be just a part of the community then. But is that possible, as some posts would be handled as comments in Drupal (I have to add the piece of information that I would be able to write some MySQL queries to read data and write it into another table). BTW currently I'm going to migrate my old s9y blog to my new Drupal site but that is just posts and comments.

So is there anyone out there who has found a cool way to either connect Drupal and Joomla or Drupal and phpBB in the best way possible?