Notably, Apple says it will inform users when their information has been requested in most cases:
Apple will notify its customers when their personal information is being sought in response to legal process except where providing notice is prohibited by the legal process itself, by a court order Apple receives (e.g., an order under 18 U.S.C. §2705(b)), or by applicable law or where Apple, in its sole discretion, believes that providing notice could create a risk of injury or death to an identifiable individual or group of individuals or in situations where the case relates to child endangerment.
Law Enforcement can make requests for Device Registration Information, Customer Service Records, iTunes Information, Retail Store Transactions, Apple Online Store Purchases, iTunes Gift Cards, iCloud Subscriber Information, iCloud Mail Logs, iCloud Email Content, Other iCloud Content (PhotoStream, Docs, Contacts, Calendars, Bookmarks, iOS Device Backups), Find My iPhone Connection Logs and Transactional Activity, Select Data From Passcode Locked Devices, and more.
More details at the link below...
Read More [via @kurtopsahl]