Apache ActiveMQ with its Ajax integration is an alternative option for widget communications to DWR; this would offer the benefit of a single messaging architecture for various kinds of inter-widget communications (wave, topic-based, session-based etc) that could enable the types of messaging to be easily extended.
On the "con" side it is pretty heavy and adds a lot of dependencies, plus it requires additional configuration in different containers (Jetty vs Tomcat etc) so would need more effort on the deployment side to get working correctly (i.e. another service to potentially go wrong). It also uses polling rather than comet, which could be an issue.
Overall I think its worth considering as part of the future roadmap.