There are many different points to consider in your question. I'm not sure where to start.
Let's start with the technical... Acrobat doesn't, as far as I know, allow a graphic image to be input into a PDF form.
Looking at the input types, you see button, combo box, etc. Nothing for an image.
And that's what would be produced: an image. The pen is just another input type.
So that leads to the next question. What is the purpose of the signature? To validate the document, make it legal? A "picture of a signature" won't do this.
You need to investigate Acrobat's "digital signature". It's a way to digitally sign a document. Meaning NOT put a picture of a signature, but rather to authenticate a document and validate it's authenticity.
So I would suggest that the whole Tablet PC/pen input thing is misleading you, and really isn't the issue.
Thomas D. Greer
Providing PostScript & PDF
Training, Development & Consulting