I was able to do it, although I had a minor issue that others may come across.
For whatever reason, two of my subforums were triggering a 404 error during the import process. It loaded the full 404 page on the screen while it did this, and at the footer of my theme was a google analytics. It got stuck in a loop timeout on the analytics. I switched themes and reimported it, and it worked great.
Two things:
1) It appears that all of my topics were imported but the users were not attached to them. I can live with this, although it's kind of awkward.
2) All of the imported topics are locked. How can I unlock them?
not sure what you mean… topics cannot be locked… do you mean forum? or is the entire site locked? if the site, you can unlock on forum – options – global settings… or if forums, you can edit a forum on forum – forums – manage groups and forums…
as to the users, did you tell the importer to import the users on the screen before clicking go?
Andy will be back tomorrow and he knows the importer better than I…
After I imported, all of my users reported that they could not reply to anything. I realised that all of the topics were locked, so I unlocked them. Rather than doing this by hand, (I have 22,000+ threads) I ran a sql command and mass changed them. So that was easy enough to repair.
The forums themselves were not locked- users could immediately make new threads in any forum.
I did not import the users, because the users existed in wp_users the entire time. I was under the impression to NOT import the useres if they already existed in the database. At no point in BBPress did it use the bb_users table.
It's cool. I've got it up and running and am working on messing around with the rest of Simple Press today. Really happy I made the jump, this system is much more suitable to my site's needs. 🙂
We map the topic lock to the bbPress column 'topic_open'. It would be useful to know if I have simply misunderstood the meaning of the topic_open column?
The users SHOULD be imported and I am currently in the process of changing the importer UI to reflect this. The process of importing them does not recreate them in WP but does ensure that the SP membership data is created.
Out of curiosity, what would the point be of not importing the users, then?
I am going to install a mirror copy of WordPress/SP and try this again and see what happens, with importing the users.
When I noticed nobody could reply, I did a search in sql to see where the topics were hiding. wp_sftopics is where they were hiding. the two topics that I manually unlocked on the front-end had topic_status 0, all of the others were topic_status 1. I did a mass replace to change all topic_status entries to 0 and everybody could reply again.
but you didnt really answer his question… what is the purpose of topic_open in your forum db? we assumed it was a topic lock… since they all were not open, we followed suit and locked…
some forums run their own user db… so you would have wp users and forum users… others like, bbpress and wpforum, for example, use the same wp user db… Andy, will correct me if I have that wrong! 
Oh, sorry. In BBPress, topic_open indicates 0 for locked and 1 for unlocked. All topics (including the locked ones) have a topic_status of 0..
Well I need to remind you this IS considered beta!
The locked topic issue is because SP does it the other way around. Zero means unlocked. I have corrected that in the latest cut of the importer. Odd that this hadn’t been reported by anyone previously who all said that the data came across fine! Oh well.
I have also changed the latest cut to remove the option of importing users. If they already exist in the target WP table then they are NOT re-imported but the actual import process ensures that their forum membership record is created correctly.
I'm wondering if there is anything I can do manually to fix the posts-with-no-users problem? Otherwise there may be some interesting disection happening in my database to reimport the users that have registered since.. 🙂
Well ideally i would like you to try the revised code if you are willing.
I am still revising but I think the bbPress side of things is working as it should be so I would be happy to send it to you on the same proviso that this is beta/in development.