This happens for me in IE6-8, Firefox 7-9, and Chrome. I set the div#RAD_SPLITTER_PANE, td#RAD_SPLITTER_PANE, and table#RAD_SPLITTER heights to auto (using Firefox element inspector) and the blank space went away. For some reason, these CSS settings are set to a number of pixels instead of auto and it is causing white space. I tried placing the webpart in different sized zones but it did not make a difference.