This is a high level overview but do your users who need to use the form offline sometimes connect to the network? I assume you are using a network or SharePoint where you can publish the template?
Once the users open the form the first time from SharePoint or your network InfoPath will cache the template on their machine. They can then open that template offline when not connected and save the XML's locally on their computer and then upload them to your server/SharePoint once they are back online.
You do need to follow guidelines for creating a template that will work well offline (for example any data connections to data sources online will not work in offline unless you build the template with this in mind (for example caching some data in the form template). Over time users will need to refersh their template by opening again when they are online.
Hope that helps. We can assist with this scenario if you like. InfoPath is great for offline usage and we have lots of customers using offline. We also have a great offline/online solution called "DBXL Client". You can get more information here" http://www.qdabra.com/en/products/DBXLClient.aspx
Ted