On his techfounder.net blog today Eran Galperin talks about one of the issues you might currently see with the date validation in your Zend Framework application - a problem in the Zend_Validation_Date component related to its use of Zend_Locale.
Zend_Date tries to validate dates according to a given format (with a default fallback). The dangerous behavior is that it tries to convert the given format to a localized format using Zend_Locale. Zend_Locale attempts to detect automatically the locale of the requesting client, and it appears that on the machines that were exhibiting the bug, a different locale was determined than those I was testing it on.
His problem was when the method tried to parse a non-localized date format and ended up converting it based on the local machine's locale settings. To correct the issue, since he didn't really need the autodetect local feature anyway, he just manually set the locale in his bootstrap via a registry setting.