I have tomcat 5.5 installed, running and verifiable at http://localhost:8080/. The Tomcat menu option appears in the Eclipse menu bar and I can start and stop Tomcat from there. In Eclipse, it does not show as a Server Runtime Environment in Window - Preferences - Server - Runtime Environments, nor does it appear in the list of environments that can be added when I click the "Add" button. All I see is the J2EE Runtime Library.
Edit:
Running on Windows XP.
Eclipse version is 3.5.1