WPF - Command and User Interface

WPF is built from ground up. After trying out different scenarious, i am starting to beleive that WPF will let us seperate the UI and business logic (provided your design is sound).

I have built some prototypes using DM-VM-V pattern. This lets us seperate the UI and business logic. However I am seeing the following exceptions.

Here is the scenario.

I have a ListView with few rows and my VM has three commands

Add
Delete
Update

1. Delete and Update works very well. Your command deletes the record from your server. Assuming VM has some kind of call back interface implemented to refresh its collection, UI refreshes itself

2. Add
When uses clicks on 'Add', I would like to bring up a dataentry window ( say NewCustomerWindow). Where should the logic of creating/displaying should recide? I do not want my command to know about this page (in addition I do not want to hardcode this window either). There must be a way out there to specify this declaratively in the XAML. I have not found it yet.