For anyone who gets the "Sync will resume when "USER-PC" is available.", and have to restart the computer and ios device, I think I figured out one of the root causes.
The normal DHCP timeout on routers seems to be 24 hours (or it is on the apple airport extreme) and for me, that's way too short for most setups... For anyone with a home network installation, the default DHCP lease time should be a full week.
I was putting my laptop in hibernate mode when I didn't need it. As soon as my computer was in hibernate mode for more than 24 hours at a time, something I often did, prefering to use my ipod for my tunes most days, my laptop was no longer on to renew its address.
The router releases the address when it has not been renewed within 24 hours, and the next time one of the devices would ask for an address, the router would swap around the network addresses. So it turns out, the ios is looking for the itunes library on computer 192.168.1.131 and the router said "nope, you're 192.168.1.132 now", and the ios device can no longer find itunes on 192.168.1.131, where it thought it was before.
I solved most of my wifi sync problems by changing the dhcp address timeout to 7 days and as long as I bring the computer out of hibernate mode at least once a week and don't need to reboot the router, my iphone no longer has a problem finding my laptop when I wake it up.
Rebooting the router causes a whole lot of other headaches and you don't want to do this to fix your wi-fi sync issues, as from my experience, that makes the problem worse.. only reboot the router when it's confused beyond all recognition and you need to restore network connectivity by doing so (and even then it's preferable to try a disconnect from/reconnect to the wan from the router admin interface first if you need to fix your internet connection).
What apple needs to do is fix the bonjour service and have it broadcast some sort of "change of address" message when it realizes the computer has a different IP address.. If the devices don't get that message and are looking for itunes and still can't find it, they should broadcast a "hey itunes computer X, where are you?" not a targetted device to device "are you there?" from time to time and have itunes always respond to the broadcast, so they can figure out where each other is at any time.
I think one of the biggest problems with wi-fi sync is the lack of address change notification between each device and when that happens due to DHCP lease timeouts, sync timeouts and other bad things happen.
At one point, my iphone tried to sync to my laptop which had been given another address, and my ipod got the laptop's original address. When I tried to sync up the iphone from the iphone settings panel, it timed out, but I later found out it managed to put my ipod into a stalled sync state that was very hard to get it out of.... and I also find the error handling during a sync operation that doesn't work or fails to work properly can cause secondary problems like leaving the itunes database in a vulnerable state that can easily corrupt it.... And if you poke it the wrong way by trying too many sync requests or attempting to cancel, then it starts to drop songs and lose track of things. When you do manage to restore proper syncing, it then realizes things are missing, only to recopy those same songs back that it thought didn't belong there from the problematic sync.
And sometimes if you poke it too badly I found it can devastate a great majority of the itunes database, then to restore the device becomes quite the headache and very time consuming.. I managed to have itunes lose nearly 2000 songs from my 11000 song library due to sync failures and I also think it managed to leave some duplicate files around, reducing the available space.
