Something similar to this (let's see if I can remember BB code).
This is the closest I could figure out how to get it. All I did is take a brush, textured it with cs_italy/culighta, and tweaked with the func_brush stuff to make a nonsolid object and translucent. That doesn't really look as good as the screen shot I saw from DoD (I don't really know where to find that specific shot, though). When I walk into mine, it disappears, and I know if I hollow it, then it'll just look ugly. So, I dunno.
*Ok, I actually did find the DoD screen shot, so here it is:
