Background: A while back, I had to replace my old Linksys router with a newer one -- the old one maxed out well below my FIOS connections 15Mbps limit (Verizon supplied an ActionTEC router, which worked quite well, but whose admin interface I hated). After the switch, iPhoto and iTunes sharing no longer worked between my Macs, or to my AppleTV after it arrived. This bothered me enough to ask about it, but I sort of gave up when I didn't get any good responses. Our recent move to a new home gave me the chance to set the network up from scratch again, but sharing still didn't work. So I set out to do some more Googling for a solution.
The answer: To enable sharing with a newer Linksys Cable/DSL Router (I have a BEFSR41 v4.1), you need to modify your router's setup. Go to the Security page (i.e. http://192.168.1.1/Filter.htm), and down near the bottom, you'll see a setting for Filter Multicast. By default, it will be disabled -- which logically strikes me as the proper setting. However, if left disabled, sharing won't work -- change it to Enabled, save the settings, and you'll be good to go! Hooray!
The solution was originally posted by NCarter in this thread on Apple's Discussions site, so all credit to NCarter!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070427133511931