I am not aware of the JPA spec ever requiring you define an equals/hashCode in your Entity objects.
JPA does require you define equals/hashCode in IdClass or EmbeddedId classes. I don't think this requirement has changed in JPA2.
A JPA provider may have additional, or less requirements. (in EclipseLink you do not need to override equals/hashCode in your Entity, or normally even in your IdClass).
If you define an equals/hashCode in your Entity, sticking to Id fields would probably be a good idea, however be careful of null Ids for new objects, you probably don't want all new objects to be equal.
If null maybe revert to identity, or just use identity (Object.equals) in general as JPA preserves object identity within a persistence context.
If you use Sets or Maps ensure your objects hashCode will never change, otherwise this could corrupt the Set/Map's hashing.