Have I been flailing on this as well.
In my case I was creating my interactive form and then using the 'Distribute Form' mechanism to publish it. This had been the first/only way I knew to make a pdf file save-able by the end user using Reader. I didn't have an 'email form' or whatever button - but that darn purple button 'Submit Form' always showed up.
Solution, duh, there is a feature in Adobe Pro under Advanced called "Enable Features in Adobe Reader..." - this allows the form to be saved with the form data. Voila.
Btw, I did add a button to email the document, but I wanted the user to be able to set the "To:" recipient. After much flailing I ended up using the following code:
event.target.mailDoc({bUI: true,cTo:"",
cSubject: "this is a subject line",
cMsg: "Please find attached the completed form."
});