A quick search (see below) showed that these are being posted all over the place:
General Real Estate Investing
Tax, Legal Issues, Contracts
Creative Real Estate Financing
Innovative Strategies
Goals, Business Plans & Entities
Private & Conventional Lending Discussion
Tax Liens, Notes, Paper, & Cash Flows Discussion
Off-Topic
Investor Psychology
Buying Real Estate
None of those is probably right or wrong; seems people try to identify the gist of what they are posting about and then find the forum that seems most relevant.
If there were to now be a forum dedicated to the topic, then maybe some of those older posts might better serve the users by being relocated to that new forum.
Search link:
http://www.google.com/search?q=site%3Abiggerpockets.com%2Fforums+self+directed+IRA&ie=utf-8&oe=utf-8&aq=t