Hi Jeff,
This could be because InfoPath thinks that you do not have permission to write back to the location from where you opened. This is similar to when you open a Word document from a SharePoint site or regular website where you do not have permission to write back into.
To remedy, make sure that the website is in your Trusted Intranet zone in Internet Explorer, and then try again.
As for forcing an unlock with DAT, there is currently not a means to do this. However, the default timeout on locked documents is 10 minutes, I believe, so it should definitely work after 10 minutes.
Hope this helps, let us know if there are any more problems!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge