Hi Chris,
You can do this although it requires custom code or our qRules product. (See this demo: http://www.youtube.com/watch?v=UXRLMTAA-2o)
However, qRules supports InfoPath 2007+ so unless you can upgrade to InfoPath 2007 you would need to go the custom code route. If you need assistance with the custom code to do this we would be glad to help and can provide consulting services to help you out. There is quite a bit of complexity in the code invoved as you have to setup a mapping between your form and the list and configure the submit properly plus you may also want to take into account certain scenarios that add complexity to updating a SharePoint list from InfoPath . For example the scenario where the SharePoint list gets updated outside of InfoPath - should the InfoPath form get updated the next time it's opened so that a user doesn't overwrite the SharePoint list data that was updated outside of InfoPath?
If you are interested in finding out more about our consulting services you can send me a private message via the forums.
Ted