Oh, that is EXACTLY it, you smarty pants - so, that was a change from IP 2007 to IP 2010, no "blank" rules. IP 2010 is no doubt removing those rules for you. What we do is have a little helper field (can be in a secondary data source, like an XML file) named "stop" - so if we need a rule where if a certain condition stops evaluation, we set stop to blank. :)
Hilary Stoupa