Currently, I have not seen any modules that currently do this. 

This can actually be a pretty tricky problem to solve, and that is increasing dramatically if you have to support a web farm scenario. However, this article (http://www.eggheadcafe.com/articles/20030418.asp) has a pretty good explaination of the issue and a solution to solve it. This could be ported to DotNetNuke but would require modifing some core code and editing web.config.

There are potentially some "shortcut" paths you could take that would not be as reliable but would be much easier to develop and would not require modifing the core. For example, if you had a module that providing a login form and a table in the database that listed the currenly logged in users (migh be able to use the UsersOnline module for this) you could check the logged in users and prevent the login. You would then place this module on a page and set that page to the Logon Page in the Admin->Site Settings. The issue with this is potentially locking out legitimate users until the session their record was clear from the currently logged in list (using a scheduler or something to that effect).

I know this was not a direct answer but hopefully this points you in the right direction. 

 Andy-
May I suggest the REPORTS module and a little imagination?  :) 
 
  
   Prevent account sharing using the REPORTS module
   http://www.eguanasolutions.com/DNN_Blog/EntryID/37.aspx 
 
 
Cheers!
-mamlin