The Bitcoin wiki says the forum started on Sourceforge, then Sirus offered to provide hosting for a proper forum. The old Sourceforge forum was then moved to bitcoin.org, and the forum was moved again in 2011 to bitcointalk.org. There are still some posts from the original Sourceforge forum preserved here. I guess Sirus must have had an account on the Sourceforge forum, and you could check it out in the old preserved posts.
https://en.bitcoin.it/wiki/BitcoinTalk#HistoryHistory
Before the creation of the current BitcoinTalk Forum, Satoshi used a SourceForge forum, which is lost. When Sirius provided hosting, the forum was moved to bitcoin.org/smf. Satoshi made several custom modifications to the forum software and theme.
Satoshi's first non-test post on the forum was:
I left the admin account set to the original SMF theme so if I somehow completely wedge the custom theme I can still get in to fix it.
I've got a neat little 12x12 coin image to replace those pip stars with. Should look nice. Also some nice button images to try.
The registration page has "hide your e-mail address" unchecked by default. I must fix that in php before we can open up.
The Announcements forum is currently moderator access only.
The forum was at some point moved to forum.bitcoin.org. The bitcoin.org domain name was also transferred from Satoshi to Sirius.
In July, 2011 the forum was moved to bitcointalk.org in order to make it explicitly unofficial. The "forum" link on the bitcoin.org homepage was made to simply return the Google search results for the search terms "bitcoin forums".