capturing digital signatures from webpages running on PDA?

[Jeroen Wenting]

We've been asked to enhance a web application we're writing for a customer to include the capability to allow the field engineers to have the customer (on-site) sign off the work order (which is sent to the PDA using a JSP/servlet application) using a signature.
They don't want the customers to have to remember a long string alternative but rather to sign using the pen on screen.
Does anyone have experience on how to achieve this?
PDAs are iPaqs running the latest OS, servers run either Linux or AIX.