I'm just tossing this out there for data points:
My toys:
MacBook Pro [Firewall ON, set to allow only specific apps, DVDPedia=allow]
iPhone 3G
2Wire wireless modem/router (The protagonist in our story) [Firewall ON, deny all unsolicited traffic to iPhone, allow only specified apps to MacBook (NO entry for the Pedias)]
When I first got the iPhone it sync'd my collection once and then failed every time after that. I tried adding ports to the Firewall, turning the Firewall off, everything I found mentioned in these forums and nothing worked.
Turns out, my iPhone was having problems maintaining a Wi-Fi signal. Surfing the web would work for a couple minutes and then I'd have to turn Wi-Fi off then on, and then I could get a couple more minutes of Wi-Fi activity, rinse and repeat.
iPhone worked fine at all other Wi-Fi locations I tested it at so I blamed my router. It was old, but I didn't think things like DHCP services or Firewalls were things that got old and tired, forgetful, etc. Nonetheless, I had my ISP send me a new router, a newer version of the 2Wire I had, and now Pocketpedia syncs fine every time. I still have Firewalls ON and clamped down with NO entries for Pocketpedia and it just works. (And I can surf the web with my iPhone until the battery runs out)
So ... no solutions or conclusions here, except to say that from my experience Mac OS X has certain desires when it comes to negotiating services from other devices, and sometimes those devices don't negotiate as quickly or in the specific way that Mac OS X wants them to.
... and that the iPhone is damn cool. And Pocketpedia rocks. (Sure, I kinda want to be able to make changes on the iPhone and have them sync back to my MBP, and be able to walk into a DVD shop and point the iPhone at barcodes, and ... but it's only 2008, Pocketpedia is free and less than a year old. I'm a happy camper.)