Hi,
I ran across the same problem as I added ultrawebgrid in the project (Infragistics2.WebUI.UltraWebGrid.v9.1.dll), and tried to fix it by the following article as suggested:
http://help.infragistics.com/Help/NetAdvantage/NET/2008.3/CLR2.0/html/Web_Adding_an_Assembly_to_a_Project.html
But it did not worked for me. All I had to do was to add the dll (Infragistics2.WebUI.Shared.v9.1.dll) in the project, and the build was successful.