Flap5
The later versions of the 737 have the CFM56 engine and therefore have a cascade style of reverser. The JT8 versions had the bucket/target style. Whether or not the reversers can be used in flight is down to two things:
1. Was the airframe / reverser combination certified using controllability (sp?) - if so, then the structure supporting the TR is stressed to take flight loads with full power deployment and flight testing has proven the controllability of the deployed TR / airframe combination
or 2. was the system cerified using system redundancy to ensure no inadvertant deployment. If this method is used, the structure is not generally designed to take inadvertant in-flight deployment loadings. The likelyhood of in flight deployment has been mitigated to such a small number by the use of a multitude of interlocks that to carry sufficient structure around is not efficient nor required.
As an aside:
Does anyone know of other military fast jets (apart from the Tornado) that have thrust reversers??