Microsoft just released an update to the Silverlight 2 DataGrid that fixes the following issues: 

• Rows containing focus no longer disappear when the ItemsSource changes
• DataGrid Selection is preserved when sorting
• Buttons outside the DataGrid work as expected when they are clicked while the DataGrid is in editing mode
• Improved FrozenColumn behavior
• Improved cell currency
• SelectedItem is updated before the CurrentCellChanged event is raised
• Many more...

Additional details on where to get the update and how to get it installed can be found on Scott Morrison’s blog.