If I make an app in PhoneGap that uses localStorage to store key/value data, will this data be synched to other devices via iCloud? I don't need to save much data.
No, localStorage is browser based and persists through the UIWebView that Phonegap gives you to play in, it's not tied to an app.
You can write a plugin to access the documents directory to save information to iCloud with Phonegap.
Old incorrect information here. See @asgeo1's answer below.