I have set:

• The project's properties to use JDK 1.5
• The system preferences to use JDK 1.5
• The JDK with which Eclipse is launched to use a JDK 1.5 installation
• JBoss to compile JSPs using JDK 1.5 (in deploy\jbossweb-tomcat55.sar\conf\web.xml)

And the JSPs are still not compiled using JDK 1.5 syntax. Example problems include
"Syntax error, parameterized types are only available if source level is 5.0"
"The type Map is not generic; it cannot be parameterized with arguments <String, Object>"

Deploying to JBoss causes the JSPs to be compiled correctly using JDK 1.5 syntax.