While Eclipse recognizes implicit objects in JSP pages, it fails to show the content assist popup for them. This can be fixed by adding the appropriate JSP/Servlet JAR files of your application to your project. If your target application server is Tomcat 6.0.x, for example, you can create a User Library and add the following Tomcat JAR files to it:
And then add this User Library to your project. Now Content Assist should work for implicit objects as well. Alternatively, if your target application server is JBoss and you have JBoss Tools installed, you can select the J2EE 5.0 libraries from the Add Library… window to enable content assist for your Web app.