I do believe, that grails users shouldn't run into problem like this.
So, I think, it would be useful to:
1. Provide a way to configure connection pool other than dbcp in DataSource.groovy not in resource.groovy.
2. Turn on idle connection eviction by default at least on production environments.
3. Migrate from legacy dbcp, to modern connection pools (I prefer boncp, but tomcat jdbc-pool is fine too).
I my opinion, grails 1.4 release is a perfect opportunity to implement such a big changes.