I have taken a look at this and on the referred topic from peteetongman there is a post that refers to the Convert How to Guide:
Available Converters List:
Taking a look at this it refers to this thread for IPB1.3.1 (the download link is broken so I searched further):
within this thread please take a look at the following post of someone who has been able to convert to phpBB :
They migrated to phpBB 2 and then upgraded to phpBB 3. They have written the steps they took to come across.
I cannot comment on whether this will work or not as I personally have not used IPB before.
Hope this helps you out. I would suggest if you get stuck further to a search on the boards as you can see others have wanted to do the same thing you are trying to do now.
Additionally I found the following link for the dead Link for the converter mentioned above located here
http://download.grenouille.com/daily-sn ... to_phpbb3/
Although I would try the method that Gardus has undertaken first and see if this works for you.
Please remember to let us know how you go so that others that wish to do this will be able to learn from your thread and understand how to do it.