I've run into an issue that, as best I can tell, is a pretty significant bug
with InfoPath.  Try the following:

1)  Design a new blank form.  Add to it a text box and a datepicker control.
2)  Set validation on the datepicker such that it will display a message or
inline alert if the text box is not blank
3)  Preview the form.
4)  Enter a date into the datepicker.  Enter some text into the text box
(which will activate validation "highlighting" on the datepicker), and then
change the date in the datepicker.

At this point, InfoPath crashes for me every time with the familiar
"InfoPath has encountered a problem and needs to close" message.  I've
tested this with both SP1 and SP2; the same behavior is exhibited in both
cases.  InfoPath crashes on at least two other PC's in our office; however,
there were two more machines for which I was unable to reproduce the
problem.  One such machine is nearly identical to mine, including hardware,
OS, software, and service packs.

So then... does this crash InfoPath on *your* machine??  Better yet, does
anybody know what causes this or how to resolve it?

-Phil