Yes, they are planning to release it. Really great news. Today there’s no DataGrid for WPF-applications as part of .NET Framework 3.5. There are only third-party controls like the Grid from Xceed or Infragistics.
Have you ever built a business application without a DataGrid? I haven’t. I’m really happy to hear that microsoft will release a DataGrid for WPF as part of .NET. That wouldn’t make thirdparty-components necessary (If you don’t need all functionality e.g. Infragistics provides). I hope the DataGrid for WPF will be more like the DataGridView of WinForms in NET 2.0, and not like DataGrid in .NET < 2.0. :-)
Take a look at the .NET 3.5 roadmap on Scott Guthries blogpost. There’s a little line containing this information about a planned built-in DataGrid by end 2008. Also some other missing controls are planned, like Calendar/DatePicker.