The EJB3 stack can be configured to use Quartz timer service by setting the system property org.jboss.ejb3.timerservice.factory to org.jboss.ejb3.timerservice.quartz.QuartzTimerServiceFactory.
Take notice though that this may not be supported in the future.
Thanks Dimitris for your quick response. I've just got one other question about the Timer Service... Is there any major reason not to migrate from version 2 to 3 of the Timer Service? It would be useful in order to choose one or another implementation for our application. Thanks.
The issue is that the quartz-based timer service has some issues with some database backends and it's not very well tested in the community. There also discussion on how to proceed with an improved timer service for ejb3.1. Until those are finalized I'd suggest to stick with the ejb2 timer service.