If you have LiveCycle ES4 (Servers not only Designer) you can add a scribble field that allows to sign a form on mobile devices.
Well, it will sadly only work if the form is rendered as html form not as PDF.
Another option is to use a Flash field that allows hand writing, but you would have to create the swf by yourself I believe.
The last option is to use a simple image field to allow users to import an image of their sign into the form.