This may be the dom-event-handler problem. One of the worst bugs in SP1. We warned MS about this before SP1 was released and they thought they had fixed it, but unfortunately, it still repros. This only happens for .NET projects in Visual Studio. Workarounds ... well, there are situations where it occurs less frequently. If you always close VS after publishing your form, it happens less frequently. Also, don't preview from VS, but publish and test the form live. Last but not least, if you suspect a problem, extract the form files and windiff the manifest.xsf. The DOM event handler section is at the bottom. If it has been removed, add it back. Good luck!
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra