@dalesaurus. Okay, I've saved out the source code of the page in question, and it's attached.
I notice that the $("#Form_Form").validate() function appears to be included on the page twice. Is this normal?
Also, there is a call to a JQuery script just above those functions, but that's SS just doing what it normally does, no?
Sorry I wasn't clear, I meant the PHP source code. Are you writing your own forms or is this from the UserForms module? I see that your date fields are not the standard 2.3.3 validated with Prototype/Behaviour. Are you using a beta version of SS?
SS Forms are pretty bang-bang out of the box, if you are having this much trouble something is wrong.