We are developing a web application based on the following system
environment:
Tomcat v4.0, Struts, JBoss 2.4.4 and mySQL v4.1.0.

Our application should deal with text messages stored in mySQL via
JDBC that may contain german umlauts (äöü).

Although we configure any software component to use UTF-8 encoding
(JSPs,JBoss,mySQL), entered umlauts do not arrive correctly in DB.
mySQL and JBoss itself deal well with umlauts (already tested).

Any help is highly appreciated.

Try updating your jdbc driver to the latest stable version, and use a
connection-url somewhat like below:
jdbc:mysql://127.0.0.1:3306/mxDb?autoReconnect=true&useUnicode=true

Thank you - this solved my problem.