It times out when pinging, but not when you open up a web browser and type the same address?
Or are you saying that when you type www.example.com you can see the web page, but when you ping example.com it times out?
If this is the case, then the DNS records are not set up quite properly.
If you're comfortable, let us know what the domain name is so we can see what the issue might be.
Indeed the server's certificate isn't trusted. That's a first hint. Couldn't hurt trying to install the certificate on the client computer, and then trying to bind.
If it still isn't working:
Secondly... Get a copy of the server admin tools for 10.7 if you don't have them already.
In Server Admin, Connect to your server and click on the Open Directory Service.
Click the Settings button at the top.
Click the policies tab
Click the binding tab.
For testing purposes ONLY and only do this temporarily...
Uncheck everything in terms of security.
Then try to connect.
After a bit of hunting online, there's another person that had the same issue with no resolution.
Until he set up a VPN. Which IMO is probably a better option for you.
Setting up a VPN will allow you to connect to your network in a secure way across the internet as though you were inside the network.
It's a bit more letwork on each client, but the security benefits are probably worth it.