This is the full error you will see in your logs at the time of submission : InfoPath cannot submit the form. An error occurred while the form was being submitted. A value in the form may be used to specify the file name. If you know the value in the form that specifies the file name, revise it and try again. Otherwise, contact the author of the form template.
There are two possible causes and solutions for this error:
1) You are trying to submit a form that has already been submitted to a document library or the form you are attempting to submit has the same name as an existing form in the library. Solution – either ensure the form is being submitted with a unique name e.g. append and date time stamp to filename specified in the Submit data connection (wizard) OR check the Allow overwrite if file exists check box, so that the form can be overwritten if it already exists in the Document Library.
2) You are submitting the form to a SharePoint list instead of a form or document library. Solution – modify the Submit data connection and ensure that the Document library field contains a URL to a Document or Form Library and not a SharePoint List.
I have been informed by an informer that it may also be due setting the form submit address to a form library view rather than just the library.
Also, another good post on this subject can be found here: http://claytoncobb.wordpress.com/2009/06/20/auto-generating-filenames-for-infopath-forms/