Since Groovy 1.8 there is support to have a closure as a parameter in annotations. A nice example how to use this feature is the GContracts project, a groovy design by contract framework, which use closure annotation parameters, to define pre and post conditions of a method.
The support of closures in annotations provides a simple way to have much more dynamic in annotations. This is cool and provides new areas where you now could use annotations. Here a simple example how a closure as annotation parameter could be used to group properties.
More details about closures in annotation see the groovy 1.8 release notes.