Lots of people have problems with their iPhones and iPads not syncing with iTunes over wifi. One thing that can stop this working is if there is a problem with IPv6 between the iTunes server and the iOS device.
The iOS device will always have IPv6 enabled but it is possible e.g. on Mac OS X to disable it.
I recently came across this problem after restricting access to Mac OS X server to only 'private networks' at the top level. This should not cause a problem but it seems to stop IPv6 working. By allowing 'all networks' the problem was fixed. It is possible to restrict access on a service by service basis. This problem was discovered by someone else but I can't find the link now.
