My son got his first iPod this weekend. It was his opening line: "Hi. I got an iPod." I had loaded it with some songs synced to my iTunes and he was quite pleased with the entire thing, even enjoying the mini games that came with. I started our evening with the simple thought of installing iTunes, plugging in his iPod and using Home Sharing to sync music between our computers. So much for simple...
The iTunes installation worked well and connecting the iPod resulting it the slightly aggravating "You have to erase this iPod" message. After doing so, I enabled Home Sharing with my iTunes account only to find it didn't see any of our shared computers. Numerous google searches directed me to the Apple technote on troubleshooting Home Sharing which is surprisingly sparse on actual troubleshooting advice. When none of that worked, I struggled with authorizing, enabled, disabling, and trying to make sure all the computers could see each other. What I discovered then is that my two other computers could see this new installation just fine while the new installation knew nothing about the originals. Very strange indeed.
I also noted that the two ports required to be open by the Apple technote were owned by different processes. iTunes itself was using the 3689 port while the mDNSResponder was using the 5353 port. mDNSResponder is part of Bonjour, which is Apple's UPnP like service for discovering network services. This led me to recall that I had once had issues using Bonjour to connect to my shared printer from my Airport Express on this same computer. After further reading, I downloaded a slightly older version of Bonjour and uninstalled then reinstalled Bonjour. Lo and behold, it worked! iTunes was now seeing both computers and the Bonjour Printer Wizard also was now discovering the printer it could not find before.
So, if you're struggling with iTunes not seeing your Home Sharing computers in one or both directions, check your firewall settings, then your authorized accounts, but then consider installing an earlier version of Bonjour and see if that fixes your issues.