Can I create surveys with unique URLs for each recipient? Does anybody know if there's a module that can support unique URLs for survey participants?

To clarify, what I mean is, I'd like to send out letters/ documentation to people in a particular geographic area. I'd like to encourage them to complete a survey, but for each recipient of the letter I'd like to assign a unique URL for their survey, which can then be used by me to marry them up to the data I already hold about them.

So, for example:

Person A receives a letter and is given a survey URL of www.mysite.com/ab123
Person B receives a letter and is given a survey URL of www.mysite.com/eg462
Person C receives a letter and is given a survey URL of www.mysite.com/qt368

and so on.

For a number of reasons, this method is better for my data collection purposes, rather than asking participants to include an assigned ID number or their own details.
You can use the webform API in a tiny module to access the form to add a hidden element to it. Then use the handy Drupal arg() function to determine the value of your unique URL argument. After, use the webform API function to change values before saving to the hidden field for administration usage later.

I can't guarantee this will work so hopefully some of the other Drupal Stack Exchange folks can verify this.