Currently there is no mention of version requirements here:
http://www.thirdlane.com/products/thirdlane-dialer

Which leads one to believe that it supports every version of outlook.

technically its old software that is free for use, but comes with no support. if it works, great, if it doesnt, dont bother to complain because it was free in the first place =)

Works with XP, does not really work with Vista (hardly anything does :)), we have not tested with 7

Works with Outlook up to 2007 - have not tested with 2010.

It is getting old and as Erik said - it is free and unsupported. That said, if there was enough interest we could revive the project, make it cross platform, etc. It is just not on our priority list at the moment.

OK, so what about the implications of the PC being behind NAT? Are there any?

plenty.. you can only use the software to send communication to thirdlane. Expect no communication from thirdlane to the app to function (popups, finding callerid of inbound call in outlook, etc)

Doesn't work with Outlook 2000 on XP Home Edition SP3.
Error on Outlook startup following: Run Time Eror '459' object or class does not support the set of elements. The standalone dialer worked, except it didn't remember the password across system reboots.

Doesn't work with Outlook 2010 on XP Pro SP3.
Thirdlane Dialer shows up in Outlook under add-ins. The first time you open an email the numbers are detected and added to the drop down. At this point you can choose one, press enter and it dials the number and rings your phone. However, any subsequent emails that you open show the list of numbers from the first email, and you cannot dial them any longer. Functionality is restored when you restart the Outlook Application but again it only works for one email before becoming hosed again. This seems like it may be an easy fix since its mostly working. The standalone dialer worked.

Alex,
Did you mean to say that it works with Outlook from 2003 to 2007? Or is it possible that there is something corrupt in my Outlook 2000 installation?

Has anyone got this working with Outlook 2000 or earlier?

the oldest version I ever used it with was Office XP (aka 2001) but at that time it had problems with office 2003. I know that it does work with office 2003 now, but making it work with newer versions may have come at a cost of some really old versions.

As far as I remember it worked with Outlook 2003 and 2007. Unfortunately each version of Outlook breaks something and tweaking was always required. It is not practical for us to support all versions of Outlook on all platforms - as you can imagine it is a testing nightmare. At some point we only supported dialing numbers from the contacts and not the email - perhaps that would have been more stable, but customers asked for email support - perhaps we could go back and build a version without email support and hopefully with more compatibility. Regardless - I would not know where to test 2010 - we don't use Outlook here.

I got this to work with outlook 2010 without any problems... I am not getting the small window to show up on incoming calls. I saw Erik said not to expect it to work... has anyone found a work around?

your only work-around is to put your PC's on a public IP directly.