Yes, all you need is WebSphere, and the million dollar license that goes with it.
On my website, I do a tutorial that creates, compiles and packages a portlet using nothing more than the JDK, Microsoft Notepad, and the freely downloadable JetSpeed 2.1. In that tutorial, you need to add the portlet.jar file (I think it's called portlet.jar..it's been a while) to the classpath of the JDK in order to compile the portlet. Then, you package the portlet application in a war file, and deploy to JetSpeed.
So, really, all you need is the JDK, a text editor, and JetSpeed, with the Portlet API jar files on the classpath of the compiler. Likewise, with Eclipse, you need the portlet API jar file on the classpath of eclipse to get portlet code to compile.
Good luck! And thanks for enjoying my free online portled development tutorials.
-Cameron McKenzie