I find that I can create a tagged PDF. I can create a dynamic PDF. But I haven't been able to create a tagged dynamic PDF.
I created a simple form in Designer 8.1, set it to be dynamic and to generate tags in Acrobat. I opened it in Acrobat 8. Under Advanced > Accessibility > Full Check, I selected Adobe PDF under Checking Option. The test failed, saying the document was not tagged and contained no fonts. But if I resave the form as a static interactive form, it passes the full accessibility check with no problems.
Accessibility is handled differently in XFA Forms. In Designer under the Window menu item there is an Accessibility menu (or hit Shift - F6). This menu allows you to define how to handle screen readers on an object by object basis. As well there is a tab tool provided that will allow you to control the flow through your form.