I have just had vinyl siding installed on my new house.
The square trim out pocket that was placed around my exterior light and gas line are "floating" not caulked in place. The builder says this is proper, but it seems to me that water/rain would run down the siding and hit the back edge of the pocket and be channeled inside/under the siding. What do you think?

There should be enough of a vinyl lip on the inside of the trim out pocket so that any water running down the siding will be directed back out at the trim out pocket. I usually install them with some loose nails but I don't see any reason that a floating piece will cause any problems as it's held in place by the light fixture.