I have had that weird error this afternoon... I cannot figure out why System.Data could rely on System.Data.SqlClient.
This happens in Mono under Linux, and I do not rely on SQL Server at all, by the way...
Oh! It says that it's only a warning but the compiler stops doing it job anyway.
Compilation Error
Description: Error compiling a resource required to service this request. Review your source file and modify it to fix this error.
Compiler Error Message: : ** (/usr/local/lib/mono/2.0/gmcs.exe:8658): WARNING **: The class System.Data.SqlClient.SortOrder could not be loaded, used in System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 ~/Global.asax