Integration with a JSR-303 bean validation provider is one of the major new features of JPA 2.0. OpenJPA has many jUnits that test this integration, but little to nothing in terms of documentation and examples.
I've been working on an OpenJPA Bean Validation Primer and corresponding sample code. A basic set of code for a sample: pom, validation constraints, validation code, domain model, and simple jUnit was recently committed under revision 951071. The primer will be published to the wiki within the next day or two, with references to the sample code.
The code in commit 951071 shows the basic functionality required by the primer, but isn't much of a usable example. A second phase of this work will include producing a more usable example.