Hey edove,
It seems like you've got quite a task on your hands. There might be several ways to do this, and I think you'll have to devise the way that will work best for you. Here's what I might do:
1. Export the spreadsheet to a comma-seperated file
2. Write a quick-n-dirty Java program to parse the file and generate all of the XML files (complete with InfoPath PIs)
3. Perform a bulk upload into the SharePoint form library
You could also work up some macros in a good text editor to replace step 2, but either way it's going to take a little bit of automation. If you wanted some help, I think we at Qdabra could probably generate the files for you an hour or two, depending on how large and complex the schema is.
http://www.qdabra.com/products.asp?cat=12
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge