- Don't use it with your iOS device. iCloud will not be required for syncing iOS devices – yet (the existing "via iTunes" method will continue to be supported), and most iOS apps would be foolish to support only iCloud-based storage of app data (especially since such data is generally stored locally on the device already – not hard to leave that support in code).
- Don't use it with your OS X machine. iCloud-supported apps like iWork and iPhoto can still use local storage, and various built-in and third-party plugins allow for other network/cloud storage services.
No one seems to have asked so far if offline setup will still be possible for iOS devices, so it's possible you will need to set up "over the air", though whether this will explicitly require signing up for or using iCloud is as yet unknown.
We should have much clearer answers to the mysteries of iCloud within the next few weeks as developers and pundits (comes to mind) wrap their heads around it.
Ultimately, like all things Apple, things will just "work better" if you use more of the integrated apps and services in your workflow/use of the related OSes/devices… Given that iCloud at the most basic level is free, there isn't much to lose in opting in (especially for iPhone users, Apple will already know so much about you, via the purchase/carrier/usage data it gathers).