I have the same problem
I'm working on a zimlet and the check boxes in IE in a user property dialog don't work at all. Other zimlets, for example Yahoo Local - the same, check box is never checked.
Internet Explorer 7 with Google Toolbar and Windows Vista. There is no trace of
js error.
Firefox works fine.
My zimbra version is:
Release 5.0.11_GA_2695.RHEL5_20081117051306 CentOS5 FOSS edition