There are probably numerous was to accomplish this, what I usually do (in WPF, I haven't done any SL development) is the following:
1. Click edit button on list page, fires an edit command in ListVM
2. ListVM Edit Command gathers necessary data and creates an EditList event (and adding the data to the event)
3. Trigger the event using Event Aggregator Design Pattern
4. ListView (or someone else) listens to the EditList event, and display a EditView page
5. Populate the EditViewModel with data fetched from the EditList event
Not sure this is best solution, but I found it useful. The Event Aggregator pattern is beneficial when communication between loosly coupled Views and their ViewModels.
Robert Blixt, Apr 08, 2011