I was wondering the same exact thing. I have a non-mobileme address for my iTunes purchases. I also use the same non-mobileme e-mail address for Facetime on my iPad. However, I also have a mobileme address that I only use for one of my e-mail accounts as well as find my phone.
Now, on my kids' iPod Touch 4th-gens, they do not have mobileme accounts, but use my iTunes acct for the app purchases and for FaceTime they have non-mobileme email addresses tied to it. For them, am I assuming that I will need to sign up for new iCloud accounts if I want all the syncing and to find them and so on?