ListBox, VirtualizingStackPanel, and Smooth Scrolling in WPF
When you uncheck CanContentScroll, you lose virtualization. And the answer is really frustrating: For now there is no out-of-the-box solution :(.
PS: This is not the first post here, asking this very question.
If you use .NET 4.5 (or 4.0 if you're willing to hack a bit) then there's an answer over here.
[Note that @Guilluame's comment was here way before this answer but it wasn't particularly visible when skimming for answers.]