I was pouring over the Silverlight 2.0 tutorial posted by Scott Guthrie, and noticed this entry which describes the currently unreleased DataGrid for Silverlight (due for a beta1 release at Mix08 in a few weeks presumably). The new beta release will also include a calendar date-picker controls (also noticable omissions from the current default set of WPF controls) as well as integral features from WPF such as data binding, and layout management.
Scott recently also mentioned a DataGrid for WPF (in addition to a number of other improvements) as part of the Client RoadMap, although without any date commitments this looks a bit further out. This is interesting, as it shows Silverlight pulling ahead in some areas of its larger and older sibling WPF. Of course there are a number of 3rd party grids available for WPF, including a free DataGrid from XCeed.