This update from the March Fastrack was just brought to my attention (thanks to Greg Amy/Scott Dowd of NER).
Adjustable seat rails for driver's seats have always been an issue. the CRB has just issued an update to 9.3.41 (page 99, the 2011 GCR) clarifying one key detail. There is now only one tiny exception to the seat back brace requirement. The number of cars which will qualify for this exception is vanishingly small. If you have sliders under your driver's seat, even if they're FIA sliders, chances are you need a seat back brace, and since it's in March Fastrack, you need the brace before your next event.
(The only exception is for FIA homologated seat assemblies in FIA homologated cars. There are very few cars running SCCA that meet that restriction.)
UPDATE if you race in a part of the country that doesn't do safety waivers (like NEDiv), and if you have sliders and are missing the seat back brace, we will have no choice but to send you home.
