Why do non-techies simply not "get" the idea of a wiki?
vivekkhurana: What you are facing is a standard problem in adaptation of technology. What most techies think usable might not be that much usable by non-techie user. As far as wiki is concerned, I find it challenging to use a wiki even being a techie. The obnoxious syntax of wiki is sufficient to scare away several users. 
 As you mentioned, people like to send email, I would suggest creating a separate mailbox for leave applications and then writing bunch of scripts to parse the email and put the email in wiki, db or whatever you want to. Second option is to use a CMS and create web form, as most CMS have support for forms and reporting built in. I have done this with drupal couple of times and the solution has been received well by non-techies.
 In the end, what ever comments you receive on this thread will turn out to be useless if you are not willing to understand the reason why non-techies are not using the wiki ? Few minutes spent with the end user will save you several hours of developing and deploying a solution that will not be used by non-technies...