I guess the reason why this question has sat here 4 hours with no answer is that we all asking ourselves "Have I missed the problem here?".
First of all you can't actually "edit every cell concurrently", after all when you hit a key on the keyboard only one control is going accept that is input, the one with the focus.
"Excel worksheet" behaviour is exactly what you get from DataGrid if you let it automatically generate the cells.
So this question is really about the object you assign to the ItemsSource
property. You really need to tell us about what you are using to store the data.
The truth is your requirement is quite easy to deliver especially if you include WCF RIA services. Ulitmately you get a "Data Context" which you can edit in various ways and then submit changes at whatever point makes sense for your application.