While Java DB is Sun's distribution of Apache Derby, we do not maintain a separate users' forum for it outside of the Apache Derby users' forum.

So, the best place to post your Java DB questions is still the Apache Derby users' alias: derby-user.

The range of questions posed and answered there are truly fantastic, and interesting, some showing a great freshness of users.

Here's a recent sample (and wherever you see Derby, you can easily substitute "Java DB"):

Access Derby Server from Java ME Client

Running Derby in an Applet Continuously

Peer Replication

Insert Update Performance

Is Derby.jar Really Built Using Javac

Problems Porting Trigger from DB2

Derby Database Directory on Tomcat