We came across a problem with InfoPath 2003 the other day. We use a lot of expression boxes to display read-only data, and happened across the fact that we can select them, type in them, and then save the data 
. All very scary when it's meant to be READ ONLY!
Only thing i've found on it is this article...which pretty much says delete them and re-add them. Did a quick test and that seems to work. If there's an easier way to fix this (as i have at least 10 expression boxes on about 100 forms) I'd be most grateful...