Check this out: http://qrdroid.com/web-masters.php
You can create a link in your web form, something like:
http://qrdroid.com/scan?q=http://www.your-site.com/your-form.php?code={CODE}
When somebody clicks that link, an app to scan the code will be opened. After the user scans the code, http://www.your-site.com/your-form.php?code={CODE} will be automatically called. You can then make your-form.php read the parameter code to prepopulate the field.