The FormFragment is currently tied to the 'change' method of the checkbox it is associated to. This causes a problem in IE6 - the user has to tab out of or click out of the checkbox before the 'change' event gets fired. Generally the desired effect should be that the form fragment shows up immediately after clicking the checkbox and not being required to cause the checkbox to lose focus.
The quick hack is to simply change 'change' to 'click' in tapestry.js inside the linkCheckboxToFormFragment method. I'm not sure if there are other negative side effects to doing so, but it seems to work in FF and IE.