I also have a little bit problems with setting the datasource.
I was used of naming my mysql-datasource mySQLDS, so I tried to use the mysql-service.xml which didn't really worked.
Next I tried to hack it up a little by surfing to http://localhost:8082 and changing the values of the DefaultDS at JBoss-runtime. I set the values as u did, only I didn't mention the username and password after the url, but instead at the principal mapping.
Maybe u can try this...
The 3.0 alpha has a bug where the CMP 2.0 persistence engine always used the default datasource. This was fixed about a week after the 3.0 alpha was released. Until the next release, I suggest naming your datasource DefaultDS.