JBoss AS 6.x requires JDK 6 AFAIK. But the source is compiled with source / target 1.5.
To change this I changed pom.xml like this:

Index: jboss-head/pom.xml
===================================================================
— jboss-head/pom.xml (revision 111957)
+++ jboss-head/pom.xml (working copy)
@@ -51,8 +51,8 @@
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
<configuration>

• <source>1.5</source>
• <target>1.5</target>
  + <source>1.6</source>
  + <target>1.6</target>
  <showDeprecation>false</showDeprecation>
  <showWarnings>false</showWarnings>
  </configuration>

To compile successfully I had to change one file:

Index: jboss-head/varia/src/main/java/org/jboss/jdbc/HypersonicDatabase.java
===================================================================
— jboss-head/varia/src/main/java/org/jboss/jdbc/HypersonicDatabase.java (revision 111957)
+++ jboss-head/varia/src/main/java/org/jboss/jdbc/HypersonicDatabase.java (working copy)
@@ -36,7 +36,7 @@
/**

• Integration with <a href="http://sourceforge.net/projects/hsqldb">HSQLDB</a>
  *

• * @author <a href="mailto:rickard.oberg@telkel.com">Rickard �berg</a>
  + * @author <a href="mailto:rickard.oberg@telkel.com">Rickard Öberg</a>

• @author <a href="mailto:Scott_Stark@displayscape.com">Scott Stark</a>.
• @author <a href="mailto:pf@iprobot.com">Peter Fagerlund</a>
• @author <a href="mailto:jason@planet57.com">Jason Dillon</a>