the only way I have found to do this so far is to use the multitouch mousepad on a mac laptop. you can purchase "autograph" from tenone, install it on the laptop ($8 i think), and then bring up a signature dialog that can be pasted directly into a container field. you have to play around with the settings in autograph, select jpg not png.
i researched alot for Macs and until Autograph came out, I couldn't find any companies selling signature capture devices for mac.
now with iPhone, you can also sign from your iPhone directly into a document on your mac but it doesn't allow you to sign directly into a container field yet. not sure if that will happen any time soon.
there might be ways to do it on a PC but i'm not familiar with them.
Andru Ziwasimon MD
I've worked with the eSign plugin from Productive Computing but their site seems to be broken at the moment.
It's Windows-only and supports specific hardware. It was fairly easy to implement.
Update: the site is working now.