HornetQ is part of the application server 7, being the default implemetnation... it's hard for us to measure who uses it... it's very widely used.
I can't disclose any customers as I'm not involved with sales... if you are interested on EAP you may find with sales guys at redHat on what's discloseable in regard to customers using it.
I have also found several users associated with it on LinkedIn.. so maybe that would give you some idea.
Thank you. This articel is very useful ...