|Unlock Forum posting with Annual Membership.|
Go to their profile and click on their wish list. Locate the book on the WL, and there should be a "Post" button. Make sure you are posting directly to their WL- if you click on anything else, such as the book description and then post, you'll be posting to FIFO.
ETA, here is the Help Center section about posting to someone's WL:
Last Edited on: 8/4/09 1:43 PM ET - Total times edited: 1