EF 4.1 does not offer a public API to retrieve DbContextfrom an instance DbSet. You will need to use the Reflections API to get the instance DbContext.
Edit
DbContext DbSet.
, , DbSet. DbContext instace .
public MyService(DbContext context, DbSet<MyClass> mySet){}
EntityFramewrk.dll Reflector , DbContext instace / DbSet. , .