There is a way to establish an Excel file to be XML/Schema based... and then you might be able to create a data connection to that file. I don't know all the ins-and-outs of it, but I'm sure someone does and has done it before.
Another possibility is this: If you have an Excel file with, say, 5 columns and X rows, and your InfoPath form has a repeating table with 5 columns, you can copy and paste the Excel data into InfoPath. The ony requirement is that you first insert the correct X number of rows in your repeating table before pasting in the Excel data. InfoPath (at least the 2007 version) will correctly past each Excel cell into the appropriate repeating table cell. It may not be ideal--but it could get you by until you find something better.
Good luck!