Una delle novità del .NET Framework 3.5 SP1, come già segnalato da Corrado, è il Client Profile:
The .NET Framework 3.5 Client Profile (Client Profile) was created to answer the feedback from many customers that a smaller framework was needed specifically for Client Applications. The Client Profile is a subset of assemblies already contained within .NET Framework 3.5 Service Pack 1. The Client Profile subset is focused on fulfilling the needs of Client applications.
In questo post è disponibile una descrizione dettagliata di cosa è e di come funziona.