In another forum I recently addressed the question of whether SS benefits are included when determining if the taxpayer must file. I started my knee-jerk "no" response, then decided to look at the 1040 instructions since I had just been preaching to a class about not working from memory.
Lo and behold, there's now an indication to include 1/2 of the SS benefit (and the $25,000 and $32,000 thresholds) in determining the filing requirement. I went back and checked 2007, and sure enough, there was a flat exclusion of SS income.
I haven't had time to research it, but I suspect that in years prior to 2008 even if a couple got the maximum SS benefit they wouldn't have a filing requirement if they didn't have other income at least equal to the filing requirement, but the combination of indexed benefits and a static threshold for taxable benefits finally pushed it over in 2008.