I'm using lastest release (10920) while previous one (10618) was working right. I get this behaviour by simply upgrading the toolkit dll in the bin folder of the web.
The matter is not culture name. I tested by using empty string, and "it-IT" culture name. "en-US" is not useful because I need italian date validation.
Furthermore the validator works right while filling in date strings. It does not work only when you press the "next" button of the wizard control. It simply mark a valid italian formatted date as invalid pressing the next button, and not while filling in it. If you remove the wizard control and do a simple postback all works right.
I think it's a bug introduced in 10920 release and that it's related to the validator placed inside another control. In fact there's a issue in 10920 release about validation wrong behaviour when validator is placed inside an Update Panel. I think it's the same bug or a related one.
Thanks
-gio