Dear John, I think your first observation is the right one, that it was God's will not to speak directly via a Prophet during the 'silent' years. But we should also keep in mind the the Old Testament is not a continuous dialouge between God and His people either. If one were to lay out the history of the Old Testament like a timeline, and mark the periods where God spoke to the People by a prophet, you would find more scilence than dialouge! <img src="/forum/images/graemlins/chatter.gif" alt="" />
Truth be know, there were many 'silent' times in the Old Testament, not just the inter-testimental period. The reason the 400 scilent years stand out is their length (a bit longer than other scolent times), and their obvious bridge between the Old and New Testaments.
God was never silent as long as the people had His Word. However, if you are a Jew, He's still scilent. 400 years was nothing, for them It's been closer to 3000 years!