We now have an importer available for Simple Machines Forum 1.1.x in the Members Area. With this you can import your categories, forums, topics, posts, users and private messages into UBB.threads.
There are a couple of things we are unable to import.
* User Passwords. Due to the encryption differences, we are unable to import passwords. After the import, your users can just request a temporary password to be mailed to them.
* Polls. SMF polls aren't attached to a topic id, so we are unable to import polls.
You can discuss this announcement here