I think he did you a favor. The last time I bought a trailer tire I asked for a metal stem and thats what they put in. I wasn't aware of any pressure ratings. As a result of your posting, I did a search for 'high pressure valve stems' and got a lot of hits. Here are a couple :
http://www.tirereview.com/?type=art&id=1851&,
http://dillaircontrols.com/.
I think we should all be aware of the matter. My TM has 15 inch tires and calls for 65psi.