Could it be that iPlanet FastTrack 4.1 does not support the servlet.jar version you are using?
I ran into a problem using iPlanet, in that it doesn't support the use of <param > tags inside of a <jsp:include > tag.
Further reading led me to discover that when iPlanet means "we support version x of Servlets and version y of JSP" that they really mean: "we support 98% of x and 95% of y".
Perhaps you have run into this problem.
I look forward to the day when iPlanet drops it's proprietary servlet runner and adopts
Tomcat (which is the rumor for the next version).