After a recent power event, I experienced this problem. None of our iPods were appearing in iTunes; even those that sync via wireless. My problem was solved on the macosxhints forums by sprale.

In Terminal:

$ cd ~/Library/Application\ Support/SyncServices/
$ rm Local

If you aren't comfortable in Terminal, you will have to use the "Go to Folder" feature in Finder since the Library folder is hidden. Just type ~/Library/Application Support/SyncServices/ in the text box and then send the Local file to the Trash.