jgulla,
You have to ensure that eclipse is actually running with jdk1.5. Go to Help > About Eclipse SDK > Configuration Details. Scroll down on the window that pops up and read the java.version= entry. If it isn’t java5 then you need to add a command line option to Eclipse to point to your jdk1.5 jvm.
eclipse.exe -vm c:\path\to\jdk1.5\bin\javaw.exe