From the mailing list:
When Cargo is used like in the email example, the context.xml has a 'user' attribute for the DataSource. However Tomcat expects a 'username' attribute.
The bug is located in AbstractTomcatConfigurationBuilder, line 96.
However, as Matt pointed out, this behaviour may be different for different versions of Tomcat. 5x expects 'username', the other versions need to be checked.