Did you notice that CRM 4.0 now validates data at the platform level? Well, it does :). Here are a couple of facts around platform validations
· They only apply to the 2007 endpoint. The 2006 endpoint remains the same (no validations to preserve back compat).
· They are implemented as a plug-in that runs at the very first stages of the eventing pipeline (before any custom plug-in)
· All validations will be enforced at the platform level (e.g. maxs, mins, lengths, picklist valid values, etc) except "business required". Why? Because business required is a UI construct and, more importantly, because preventing records from saving if an attribute value is missing would break many ISV solutions (e.g. an attribute may be filled with a plug-in or a workflow down the line on the pipeline)