Wildshaping is not specified to allow you to escape a grapple, and therefore, doing so will not actually break an existing grapple. However, turning into something unpleasant to grapple with may be enough to convince your opponent to voluntarily relinquish his grapple attempt, or perhaps give you the upper hand in an ongoing grapple, perhaps changing it so YOU want to grapple, rather than your opponent, particularly since the natural weapons of an animal form tend to work just fine while grappled.
For those high-end wildshapes, turning into a fire elemental is amusing, too.