I had to add "Infragistics2.WebUI.WebDataInput.v7.1.dll" in the Bin directory of my SharePoint site before my web part would show up. My web part contained four linked WebAsyncRefreshPanels. I did not have to have that DLL as a reference in my project or loaded in the Bin directory of my non-SharePoint test site. Just thought that was weird, and I wanted to share this info in case anyone else runs into the same problem.