in long surveys people get bored and tend to drop answering...
could there be a temporary pseudologin function?
on closing unifinished window, a dialog would appear proposing the pseudologincode
that can be entered later to continue the survey..
the respond could optionally enter his own code to be easier to remember...
more about this issue http://www.limesurvey.org/index.php/Can-I-do-this-with-LimeSurvey-/20024-anonymous-with-GeneratedCode-to-let-continue-later.html
How about a php-script at the beginning of the survey, that creates a user for eg. 3 days, gives that info to the person taking the survey and then autologin the user? It's not something I'm going to write now, but seems very easy to do.
Franky