pio!pio!: SSBI (Special Scope Background Investigation), if I'm not mistaken, came about when they combined the old BI (Background Investigation) and SBI (Special Background Investigation) classifications. I think they only go back maybe 10 years or so now. I'm not sure, but either AndrewR or UltraQuiet might know the full story behind the revamped clearance thing.
CrazyDe1: To answer your question, it depends upon the misdemeanor. When I first obtained my TS-BI back in '85, some misdemeanors were disqualifying. Parking and speeding tickets were not. Above-average indebtedness and substance abuse were not favorable.