I keep getting asked this question after successfully migrating my 1.2m post forum from YaBB to SMF2, so I thought I'd detail the steps, which hopefully we can refine as others go through the same process.
Firstly, the gotchas:
1) My site never supported attachments, so that is not covered here (yet)
2) Internal (or external) links are not converted - I got around this by keeping my YaBB site at
www.omegaowners.com/forum/YaBB.pl and my SMF at
www.omegaowners.com/forum/index.php - although they share some directories, they don't share any files, so can coexist. Robots.txt and sitemaps slowing being tweaked to put more emphasis on the SMF pages
3) Membergroups, Boards and Permissions all need to be rebuilt. I'd recommend understanding the permissions on SMF by setting up a test site, and playing around with that long before migrating
Put your YaBB 2.5AE site into Maintenence Mode.
Run all the YaBB Maintenence Controls (vital).
Make a backup of your entire YaBB forum.
Install SMF1.1.14 Full version, when creating the admin user, use a name that does not exist on your YaBB (important). eg, admintemp.
Upload the YaBB2.4 to SMF1 converter, along with convert.php
Run the YaBB2.4 to SMF1 converter.
When this is complete (took about 4hrs on my 1.2m post forum), you need to rebuild the Categories and Boards (which probably didn't import properly).
Recreate Membergroups if they didn't import.
Recreate Permissions, and check they are correct.
Run the SMF Maintenence tasks.
Upload the SMF2.0 Large Upgrade.
Run the upgrade (this took about 20m on my 1.2m post site).
Upload and run the SMF2.0 to SMF2.0.1 Update.
Check the migration looks valid and correct and complete.
Delete the temporary SMF Admin User (admintemp?).
Its possible that you could go straight from SMF1.1.14 straight to SMF2.0.1, but I have only tested going via SMF2.0
Hope this helps others who feel stuck in YaBB, and queries, lets keep it to this thread so we have all the info in one place.