The xbox saw much of its sucess from modders. It seems at least half the people I know got xboxes simply to mod them and would not have otherwise got them. Also those people generally own about 2-3 xboxes a piece because they wanted to try different case mods or because they got banned from xbox live and had to get a virgin box just for online play.
I think the 360 needs to be able to be modded in order to see the success that the xbox saw. If it cant be modded its dead. If people figure it out right away and modchips are available within a few months, then it could end up being #1.