Michelle,
I decided to have another look after getting your post, as it was driving me nuts.
Yesterday I downloaded the very latest Silverlight API (2.3.0.774) in the hope that the problem had been fixed, or to see if my previous installation had been corrupt in some way. The same problem exists with the new version.
I have come to the conclusion that the error is not related to the parameters being passed (Name and Extent), but is being triggered by something else that happens inside the DLL when the AddBookmark method is called.
I tried without success to set as many of the Bookmarks class properties as I could, to see if the null reference was down to them.
I eventually experimented by adding a physical bookmark control back in to the XAML (previously I had just created a new Bookmarks class object in code - which used to work with version 2.1). After adding the object to the XAML, and using its name as the reference instead, I can get the code to work again. I tried to set the visibility property of this object to collapsed (to hide it) - but this re-introduces the error condition. This leads me to think that the API is trying to do something to a physical bookmark control, even if it is not there!
I then tried to set the width and height of the physical bookmark control to 10,10. This also causes error, as I think the bookmark list is not visible. Eventually, this code will now work - notice that the height has to be large enough to encompass the list region:
XAML
<esri:Bookmark HorizontalAlignment="Left" Margin="0,0,0,0" Name="gblBookmarks"
VerticalAlignment="Top" Width="0" Height="100" Visibility="Visible"
Map="{Binding ElementName=Map1}" UseIsolatedStorage="False"/>
VB
Private Sub AddBookMark(ByVal Name As String, ByVal XMin As Double, ByVal YMin As Double, ByVal XMax As Double, ByVal YMax As Double)
Dim extent As New ESRI.ArcGIS.Client.Geometry.Envelope
extent.XMin = XMin : extent.YMin = YMin
extent.XMax = XMax : extent.YMax = YMax
gblBookmarks.AddBookmark(Name, extent)
End Sub
Hope this helps, not an ideal solution I know, but at least I can now start using the other new features in the latest API version.