You cannot log onto ICO simultaneously with multiple machines using the same serial. LAN play is however permitted.Which is awesome.I play it over LAN with my nephew on a daily basis.
You realize it is PERMITTED, and not really moral, right? If you are playing daily on two machines with two people, you really should do the right thing and have two copies.
I think they permit LAN games to allow people to try it out, and to allow people to have LAN games without hauling their own personal PC to their friends place.
My friend used my copy of SINS to play on the LAN with me for a few games, then when he realized he liked it, he ordered his own copy. (FYI They are sold out everywhere in Edmonton Alberta LOL). Its the right thing to do since these guys are such a small company compared to the larger ones.