Ironically enough, UL really doesn't care about lampholder orientation, with two exceptions:
First, they really frown on outdoor fixtures being constructed so as to accumulate water in the socket, or for cold rain to hit a hot bulb. So "bulb up" is an issue with wet location lights.
The other concern is heat. There are many, many fixtures out there that can be mounted in any position EXCEPT bulb DOWN, as the top gets too hot and melts the socket. You will likly find this restriction on the cheap "bulkead style" fixtures sold at the box stores.
A "side project" of mine is constructing a replacement for this towns' Hannukah menorah. One of the problems with the existing one is that the 'bulbs up' arrangement makes it an electrocution waiting to happen every time it rains!
