As this thread shows, there seems to be an issue getting JSPs to compile in GWT hosted mode with the new Jetty server:
2. ERROR in /tmp/Jetty_0_0_0_0_8080_war___
This problem is generated by the default values used by JspServlet, which compiles using 1.4 for source/target values.
You can cofigure this servlet by adding
<servlet>
<servlet-name>jsp</servlet-name>
<servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
<init-param>
<param-name>compilerSourceVM</param-name>
<param-value>1.5</param-value>
</init-param>
<init-param>
<param-name>compilerTargetVM</param-name>
<param-value>1.5</param-value>
</init-param>
<load-on-startup>3</load-on-startup>
</servlet>
I described the solution at http://code.google.com/p/raisercostin/wiki/GwtEclipsePluginDebug too.