Recently I encountered a situation where a linked item would only show in preview mode when the preview date / time was prior to 12/15/09 12:00pm.  There were no publishing restrictions on my linked item.  So I was very perplexed.  After a few fruitless days of exchanging posts on the support site my issue was escalated to the next level and I was able to talk to a support rep.  After a few minutes of showing him what was going on, he figured out that it must be a publishing restriction on an item in the path of the linked item.  So we walked back up the item path in the content tree checking publishing restrictions for each node as we went along.  Finally we got to the Home node and low and behold someone had set it to expire on 12/15/09 12:00pm.  After we removed that expiration, previewing worked as expected. 

 HTH!