DVN, I'm not sure on this one as I have not experienced it. Can you make a simple test form with one of the failing data connections and see if that also fails for those users? Can you see if those users can open it on a non-Vista machine? Do you have any errors in the event logs or in the 12 hive logs related to InfoPath?
As for the rules stuff you figured out, I'm glad to have helped. Do you mind going back to that thread and updating it with your final solution so that others can learn from it and so that we (those who helped) can see what you ended up doing? It is as helpful to us for you to explain your final solution as it is for us to help you get there, because that helps us understand what information and what explanations help the most. I'm most curious as to whether you utilized my blog entries directly, if the blog entries helped you understand what you need to do in a different way, or if you just figured it out completely with a different method. Any feedback would be helpful to the forum and to me personally. Thanks!