We are using ejb3 in production (have been since half july).
You do risk some API changes between versions of the specification, but the semantics are basically the same. Things should really settle out near November when the EJB3 spec goes final.
EJB3 persistence is a very very thin layer over Hiberante 3.x which is being used in production in many sites. The Session/MDB layer is based JBoss AOP aspects which have been around for a few years.
Really, most of EJB3 was mapping new metadata types (annotations) to internal meta structures.
Don't know if that makes you feel any better :)