So there I was -- another day of writing slick AJAXified custom SharePoint web parts. This was a simple one. A series of dropdowns that cause subsequent dropdowns to be re-populated with increasingly restrictive data. Planet Continent Country etc. But, when I changed the planet choice to be "Earth" the continent choice DID populate with the continents on earth but it caused a complete page round trip -- not the smooth AJAXified action I expected. Plus, my UpdateProgress panel wasn't being displayed...