Updating is slightly more difficult. You have a few options, including:
1. Handle an Updating event as shown in the book. This is relatively easy.
2. Hook into the SavingChanges of the ObjectContext, find the entity being updated, check if its of the type you're expecting and then set the UpdateDateTime.
The process is explained here: http://msdn.microsoft.com/en-us/libr...ngchanges.aspx
That example doesn't update a property, but validates data, but the concept is the same. At the location where the validation takes place, you could assign the date and time to your property.
Hope this helps,