It's on page 73 of Unearthed Arcana ... there's a thread recently started on this.
Basically, the medium BAB classes (rogue, scout, cleric, etc) get 3/4's of a BAB per level, poor BAB classes get 1/2 of a BAB per level, and the good BAB classes get 1 BAB per level (as normal). Add up what you get from your various class levels, and that's your BAB. IE, a scout 1/rogue 1 would have a BAB of +1, +.75 from scout and +.75 from Rogue for +1.5, but you always round down. A Rogue 1/Sorcerer 1 would have a BAB of +1, +.75 from Rogue and +.5 from sorcerer for +1.25, rounding down.
Nuts, beaten by Salthorae.