Peter Doornbosch has created a much improved version of the JTop sample plugin for JConsole. JTop only shows thread name, cumulative CPU time, and thread state. The "top threads" plugin also shows, per thread, current CPU percentage, average CPU percentage, and the evolution of CPU percentage over time. Very nice!
There does seem to be a bug whereby the TextArea that shows the stack trace of the selected thread is editable. Furthermore this TextArea is "always on top" so it can obscure part of the "new connection" dialogue. I found that I could shrink the TextArea down to nothing by grabbing the control at its top, so this problem isn't very severe.
Thanks to Daniel Fuchs for the pointer.