I've been dealing with Outlook 2007's search problems since installing it way back then. Most frequently, I'd search a keyword; the search pane would fill with information, and then disappear leaving me with "No results found." I can replicate this problem on every workstation in office, as well as my home workstation.
Well, Microsoft has announced the RTW of Windows Search 4.0 as of Tuesday June 3, 2008 and my preliminary tests look promising. So far, all of my keyword and "from:" searches have worked as expected with no clearing of results.