You only need to create a new rule for the the Lync auto discover service.  The full functionality uses just the regular External Webservices URLs.

For more info on the previous steps go to this article:
http://www.pro-lync.be/blogs/lync2010/archive/2011/12/09/first-lync-mobility-installation-overview.aspx

Create a new Web Publishing Rule

This is obviously an allow rule

Depending how you load balance or whether you have standard/enterprise edition this might be different on your end.
ISA/TMG supports coockie based load balancing (requirement for external web services URLs) if required in your deployment

You have to select SSL to the server/loadbalancer

Enter the name of the webservices loadbalancer or pool

You have to select to forward the original host header

The public name is lyncdiscover.<domain.com>

Select the correct listener (preferably both port 80 & 443)

Select authentication settings (Depends on your security requirements)
This setting is different from Microsoft recommendations "No Delegation, and Cannot Authenticate Direclty"

Remove All Authenticated Users and replace this with All Users (No Authentication)

Finish

Now you have to change the following tab on the rule

You have to include both HTTP and HTTPS and forward to the correct ports

HTTP    => 8080

HTTPS => 4443

Change the authentication option for HTTP on the Listener.
(you might need a new listener for security reasons so that the listerner never sends authentication)

Publish your rules and test…