The key to getting it working is discovering your Packet8 username and password. The DTA310 will never reveal to you the password, however it can be made to reveal the username. If you install the packet8 "unlocked" firmware and then go to the SIP settings page it will show you your 10-digit username. [While you're in the unlocked firmware go ahead and disable the G729 codec]..
How do you get the password? The password turns out to be the original 10-digit activation code you received from Packet8 when you first ordered service.
Now you can configure your favorite VOIP software (I've been using X-Lite) for SIP authentication with Packet8. Set the domain to "packet8.net" and for the SIP Proxy (as of this writing) use 126.96.36.199:15062 or 188.8.131.52:15062. If the address ever changes the unlocked firmware will show you the current server it's using, so just check with your DTA310 first.