I would like for Reading List to automatically sync between an iMac, MacBook Air, and an iPhone 4S. The Macs are both running Lion 10.7.2 and the iPhone is running iOS 5.0. All three devices are signed into the same iCloud account and have 'Bookmarks' checked. It appears that some syncing has occurred at some point between them, but it hasn't seemed to be working lately.
Sometimes this can be caused by temporary issues with your account or the connection to the iCloud servers. Waiting it out isn't a bad strategy if you don't want to go for official support or when restarting the affected devices is not effective at clearing up the issue.
For ongoing issues, you might reach out to iCloud support to ensure it's not an issue with your setup or account.
This might be an issue with Apple ID password requirements. Does your 8 character Apple ID password include at least 1 number and 1 capital letter? If it doesn't, try setting a new compliant password to see if this fixes the iCloud problem.