I know it works with imp. It might also work with familiar and I havnt played enough to try any other summons.  I found it in the last game, started a new one just to make sure.

Here is turn 15: http://screencast.com/t/ZWM4N2UwOW

that lower space of visible land, in which I have no pieces, are from summoning Familiar. It, however disappeared after about 5 turns, although it might have been from me walking through it.  To test imp I summoned one where I was standing there, and then went to my city.

Screen Turn 70: http://screencast.com/t/OWEyMGQ2Y2

55 turns later, I still have full visibility on that area, smaller than the Familiar but I can still see the opponents moving around. 

Screen Few Turns Later:http://screencast.com/t/NzkzZDk3NWU

I then wandered through the visible FoW, in the thought that perhaps that might cause the mechanic to begin working correctly. This did not happen. As you can see in the screen, the visible land was still visible.

Again, figured this was not how the game mechanic was supposed to work, so I'm just pointing it out. Also, do you want the logs for this? Ive just been dropping the screenshots so far, because Ive been continuing to play. Just let me know one way or the other, Ill try to back them up till then.

Cheers,