This happened when I upgraded my iMac from 10.6 Snow Lep to 10.7.3 Lion.
This worked for me.
You need to edit the hosts file.
To get to the "hosts" file
1. Under "Go" menu in the finder choose "Go to folder"
2. type /etc (this opens a new finder window)
3. Find the "hosts" file
4. Option drag the hosts file to your desktop to copy
5. Right click or CTL left click the copied hosts file and Open with TextEdit.app
6. Delete any lines of extra info (the info should look like this...)
# Host Database
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
7. If your file already looks like this, save the copy with no extention (no .txt) as hosts
8. In the etc folder where the original hosts file is, rename the file as "Old hosts"
9. Drag the newly saved hosts file on your desktop into the etc folder
The OS will ask for passwords to authenticate during this process.
When that is finished, open the Terminal.app in the Utilities folder.
At the prompt type
to clear the DNS cache
Restart your OS and open FaceTime.
Login and it hopefully will work for you.
In addition, I read to add 220.127.116.11 and 18.104.22.168 to your DNS server list in the Network Preferences in the Advanced settings.
I don't know if I needed this but FaceTime now works in Lion.