Hi USAComp2k3 and welcome,
Greg Collins recently blogged a technique we use to send around forms: http://www.infopathdev.com/blogs/greg/archive/2009/04/23/create-a-split-public-private-form.aspx. The key thing here is creating two versions of the form. The first version is a restricted URN based solution and you just attach the XSN and send it. Then, you take the same form and upgrade it to be your SharePoint form (but keep the name the same as the URN one). So, when your staff opens XML that are sent from the external field, it downloads the more recent SharePoint published version. Now when your external folks open an XML, it will find the old version and present a warning. Should not be a problem though so long as there were no incompatible schema changes between versions.
Good luck! Please let us know your success here. We have this working in house. If you want to donate some $ to have us work with you directly on the problem, just donate what you think it's worth in terms of your time multiplied by your hourly salary and we'll contact you. I'm sure we can do it for cheaper than you can... Here's our PayPal donation button: https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=5038435.
Otherwise, the forum is ABSOLUTELY FREE. We'll try to respond quickly. Thanks for your patience.