Interesting for me as well as my company SOP allows selection of speedbrake up to flaps 10, but not at flaps 15 or greater. Thats classics only though and might be different for the NG (they had some speedbrake problems in the beginning on the NG iirc). In our AFMs is just a recommendation to not use speedbrake at 15 or greater, but it is still allowed according to the AFM.