While configuring Reporting Services on a server running Windows 2008 R2, I came across this issue. After some searching I found out Microsoft has a kb article about this (http://support.microsoft.com/kb/972328), however it mentions only applying to MSSQL 2005. Well the Server I had had MSSQL 2008 R2, as well as SharePoint 2010 and Exchange 2010. It seems that this fix also still works. So if you are confused as to why this is happening and think the article doesn’t apply to you, give it a shot anyways.
Info Path 2010 does not let you change the field type to a fill in or any other type. You have to use SharePoint to modify the column. However once you do this, even when you reopen the modified form in InfoPath, the drop down does not change to a Fill-In type. It seems while the column is updated (and InfoPath asks to update the changed columns), it doesn’t update the binding in the form.
In addition it seems that InfoPath 2010 doesn’t allow any option or checkbox to enable Fill-In type. The only way around this is to delete the control field dropdown (not the field) and add the new one (by dragging it in). Not the friendliest of setups when you have rules and other changes to the control.