as long as you warm it and not bring to the boil its fine. For me last time i immerse the breast milk storage bag directly into a bowl of hot water to warm it about 15 mins or so before i empty it into the bottle to feed my boy.
in case you need more expert advice: (extracted from Storage and Handling of Breastmilk)
Another, newer method that affords a faster warm is the Avent bottle warmer. This warmer allows for a gradual warm and has been tested on breastmilk and not shown to affect any of the milk's properties. Regardless of the method of warming you choose, shake the container before testing the temperature. Human milk should NEVER be microwaved or heated directly on the stove! These heating methods alter the composition of the milk and may result in hot spots in the milk that could perhaps burn your baby!