I happened across a wee bug in Views 2 today. I think I've noticed it before, and I'd be a monkey's uncle if it isn't already in the queue, but this is a synopsis:
If you expose the Node Type filter in Views 2 and set it to "Optional", but also with "Limit list to selected items" checked, you might expect it to continue to restrict the results to the selected items it refers to, right? (Well I did.) It does not. If your use selects the "< Any >" option from the resulting UI they will get *everything*
Doh!!