iCloud Keychain has been something we've considered but currently we're focusing all our resources on integrating our apps with Fuelly.com, so we haven't implemented the feature yet.
We'll revisit the idea once we have our apps in a position where we feel comfortable rolling out new features.
Yep, that seems like the sensible course of action!
When you do get to that point, if you are already using the standard system keychain SecItem to store/retrieve credentials, basic support should involve just adding that one attribute to the SecItem add.