I just noticed that the bug I first time saw in the first beta version of 4.0 is still present - see this forum posting and especially this one. If you have a wizard generated tabular form with a select list and you hit the F5 button to reload the page, your select list values will change. This happens only if you use FF. With IE instead it works fine. See this example:
If you are already using 4.0 in production, you should do something to avoid unexpected results. I am sure that the Apex-Team will correct this bug with the first patch for 4.0.
The actual workarround, as posted in the second thread dealing with this isssue is to switch the "Autocomplete" to "off" in the page attributes.