On a site with premium user account subscriptions, I'd like to be able to limit logins to one computer at a time. The most straightforward way to accomplish this would be to limit by IP, but I haven't had any luck finding a plugin to accomplish this. Does anybody know of one I can use to get this functionality?
As already suggested in a comment above of mine, technically a (cookie based) session is limited to one IP at a time by using the Safer Cookies (Wordpress Plugin).
This does not prevent to allow another login with the same username and password because it is a feature of wordpress to allow you to login multiple times with your credentials and therefore with multiple IPs.
What you would like to achieve is to prevent multiple logins at all then. According to Otto, this is not trivial. Stacy writes, that the Wishlist Member (Wordpress Plugin) has such a functionality built in: IP-Login Protection. So probably that plugin or part thereof is a solution to your issue. IANAL, but that plugin is violating the GPL license as it restricts you to make use of your wordpress derivate. I would not use it.
But probably there is another plugin out there available, or it's quite easy to extend Login LockDown (Wordpress Plugin) to bind the whole session to the last valid IP recorded.