Acrobat 7
Adobe just released Acrobat 7. Acrobat Pro includes the form designer, which is what I work on now.
I cooked up a form to try it out, and put it online. Acrobat 7 supports allowing Reader to submit forms via email, or by saving the XML to disk. It also lets you sign and certify documents.
So if you click here, you'll get a PDF that you can use to send me an email. It's signed by me on my own computer using a key I generated myself in Acrobat, so Reader or Acrobat (whichever you use to read it) should tell you not to trust me. :)
I didn't try it in any version of Reader before Reader 7, so I don't know what will happen if you open it there.
If you type some stuff into the form and save it, you can see the XML it generates. This is cool - you can have forms that make SOAP calls when you Submit, which can make PDF an interesting UI in front of a web service.
This is all pretty new to me.