I tested the described scenario with Q1 2008 and Q2 2008 versions of RadControls and RadPanelBar behaves the same - when NavigateUrl property is specified, the page is redirected and ItemClick event is not fired.
I suggest you set a custom attribute
instead of NavigateUrl to the panelbar items and store in it the value of the NavigateUrl, in this case ItemClick is always fired and in its handler you can retrieve the NavigateUrl value and redirect the page manually.
Please check the attached sample project for a reference.
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