As per http://support.microsoft.com/kb/922672 there is a specific set of assemblies supported inside the SQLCLR i.e.
If a user assembly references any .NET frameworks assembly that is outside of the ‘blessed’ list and you try to register the assembly with SQL Server, error 10301 will be raised.
Server: Msg 10301, Level 16, State 1, Line 1
Assembly 'AssemblyName' references assembly 'accessibility, version=188.8.131.52, culture=neutral, publickeytoken=b03f5f7f11d50a3a.', which is not present in the current database. SQL Server attempted to locate and automatically load the referenced assembly from the same location where referring assembly came from, but that operation has failed (reason: 2(The system cannot find the file specified.)). Please load the referenced assembly into the current database and retry your request.