My 13" Macbook Air recently started showing the "service battery" alert. It's the mid-2012 model, and is still under Apple Care. The alert seems to appear mainly when I'm running off the battery. Is this a battery issue or a bug in the system?
You don't say what version of OS X you're running, but if it's 10.9, note that other users have reported a change in battery alerts after upgrading to that version. http://www.zdnet.com/macbook-users-seeing-service-battery-message-after-mavericks-upgrade-7000022588/
Open Utilities > System Information, click on "Power" in the left column and see what stats show up for your battery. This will be useful when talking to Applecare.