I Have had this problem for ages, and I have found something that has so far worked for my iPhone 4 and iPad so thought I would share it, I'm sorry but I only know the windows version of this but I'm sure mac users can adapt it.
with iTunes running do a cntl-alt-del and choose task manager and then click on the services tab, scroll down the long list till you find apple mobile device, right click on it and choose stop service, or pause service,, but DO NOT delete it or you may not be able to sync your device at all, now close task manager and close iTunes, restart I tunes, plug your device into a power supply and it should come up, has worked for me for a while now.
hope this help and has not been suggested before,, way to many replies to read them all.
Forgot to say, connect device my cable one last time and untick and tick sync by wifi for that device....
Message was edited by: DataUK