Hi There,
We will start charging for hosting the service pretty shortly, although there will always be a free level of usage - for people who need more capacity, they'll be able to upgrade to a paid account.
We definitely don't require users of apps to get an Acrobat.com account - you can read the Developer Guide PDF, section 6.4, included in the SDK Navigator (see the "Documentation" tab) - for more details. This way, you can authenticate users against your own backend systems.
If at some point you need to start paying for using the service (say, you get a ton of users), you'll probably need to upgrade your account, and pass on the costs to your users in some way.
hope that helps,
nigel