I had a similar issue with a server based form giving this popup on opening of a saved .xml
"This form cannot be filled out either because you do not have adequate permissions, another user has the form opened or the the form is open in another program.
Do you want to open a read-only version of this form?"
I didn't want the end user to overwrite the .xml so the permissions on the server were Read - only. In order to keep the above from occuring, I set the individual files permission to Read-Only, and infopath(2003) now happily opens it in READ only mode without a popup.
It also allows multiple users to open the same form simultaneously as they don't put a file lock on it upon opening.
This might work for you, or not, it worked for me.