What is this small hole in the face of my propane regulator?

I've stopped using the propane system until I can understand why
there's propane leaking out of that hole!

http://mira.sbw.org/photos/20120528/P1000510/

More photos:

http://mira.sbw.org/photos/20120528/

Van Cafe in Santa Cruz installed this new propane tank, valve, and
regulator in January, 2011.  I've used it only a few times since.

Last time I used it, when I opened the valve, I noticed a hiss.  I
located the leak with my thumb.  My nose and my electronic propane
sniffer confirmed it was propane, so I shut the valve and decided to
wait until I can find out what the deal is!

Is this an overflow vent of some kind?  If so, is there a better way
to vent the tank/pressure down to the correct level?  Sounds kinda dangerous.