I'm going to start working on a game that will probably have local multiplayer built in.  I'm considering putting Live! networking in, but I don't know if I'll be able to get around to it.  How difficult would it be or what problems would come out of putting Live! networking in at the end, rather than building it like that from the start?  Is there anything I should do now to make it easier later on?  Since I'm not absolutely sure I'm even going to put in Internet Networking, I don't want to put a lot of time into it, only to find out it won't be included in the final game.

The code itself shouldn't be hard to be implemented later. But I think if you structure your code properly, from early on with networking in mind, you'll make things easier.

I would have to agree with it not being too hard. It will take a bit of time (depends on the amount of code, complexity, required latency compensation, networking background, design, etc) but it shouldn't be too bad. For the most part you will just be copying logic over to the server and stripping some out from the client (depends on what it is and how you want it to work) for the basic structure.