Seems as though the "destroywi" command has been moved from the Power Tools into the actual TFS administration executables. I case you were wondering here is the /? from the new destroy work item command in TFS 2010.
C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE>witadmin destroywi /?
Work Item Tracking Administration tool. (C) Copyright 2006 Microsoft Corporation. All rights reserved.
Destroys a work item or work items. Currently, work items can only be destroyed. Destroying a work item means the work item is physically deleted and cannot be restored. The system does not support logical (and thus recoverable) deletion of work items.
Usage: witadmin destroywi /collection:collectionurl /id:id [/noprompt]
/collection Specifies the Team Foundation project collection. Use a fully specified URL such as
/id Specifies the ID of the work item to destroy. Supports multiple IDs delimited by commas
such as 12,15,23 (with no whitespace).
/noprompt Turns off the confirmation message.
I was looking for this using the old "tfpt destroywi" command and kept getting errors saying the command didn't exist when I finally did some searching and came across the new home for the command. Could be pretty helpful if needed.