From Namco Bandai:
Due to several contacts we've received from users confused as to how the summoning feature works in game, we have provided this list to help.
Conditions for summoning (White Sign Soapstone):
The host (person who's game world will be used) must be human. The client (person joining the game) can either be human or undead.
The client’s level must fall within 10% (above or below) of the host’s level. Generally, this example formula should give you a good idea on whether you will be able to join: (LC x 1.1) ≥ LH ≥ (LC x 0.9), where is LC the level of the client and LH is the level of the host. For example, if the level of the host is 20, then they will only be able to summon players who are levels 18 to 22.
The area Boss in the host’s world must still be alive. A client can still be summoned even if the client has already defeated their own area boss.
The host must not have placed down a summoning sign. Summoning signs from other players will not appear in a host’s world if the host also has placed a summoning sign down. This is to prevent a network conflict of the host joining someone else's game world while the clients are trying to join the host's game world.
After successfully summoning, the client will return to their world if the area boss is defeated, the client is killed or the host is killed.
Also, please note that there are several different servers/lobbies that are being used for Dark Souls, so you may not be able to summon a specific friend into your game even if you adhere to the steps above. We can only recommend that you follow the steps above for the best chance at being able to join a specific friends' game.
Pulled this off of a Wiki, says its from Namco.
So to all those hollows running around out there...drop your sign and you might get summoned.
I have my soapstone ready to use, i put it down when I enter a new area. Kill a few zombies and then place it again. Mind you that has never got me summoned,, but the game is much easier if you have a partner or two.