Only time I've used a digital sig was when I refinanced the house - due to fax problems, I had to do the good faith estimate via email. It was sent as an Excel spreadsheet, I copied the whole mess into another worksheet, and used my PGP key to "sign" the clipboard contents. I also put a note on it to the effect that I'd sign in ink when we met in person. The loan agent didn't have a clue, but the bank manager accepted it.
I think for your client you would need to be able to capture the IP, etc. so that you could go back to the ISP and prove that at that time he was connected via that IP, etc.
Since IANAL, I'd ask one.
