One fantastic feature of Silverlight is that you can "virtualize" the UI objects needed to render a list, reducing the amount of memory and CPU necessary to load large lists. So you can bind a ListBox to 1.000.000 items, but you create only ListBoxItems for the first 100 items. This helps you to make your Windows Phone 7 App a little bit quicker if you want to work with large data lists.
The only thing you have to do is to derive your data source from IList. This fact triggers Silverlight to do data virtualization.
More informations about this feature you can found at Peter Torr’s Blog: