Two different official Apple USB cables: 30-pin & Lightning
 Same 30-pin cable shows iPod classic
 Different USB ports: one on the front of the Mac Pro, other via Apple display (rear Mac Pro port)
 None show up via WiFi either
 Works okay with Xcode via USB
 Restarted iPads, updated/restarted iPads
 Restarted computer
 Works okay on other computer with iTunes 10.7
Ahh, now that I started whinging about it, I've started thinking more rather than just being frustrated.
I remember having to update Xcode & the iOS SDK recently because I updated the iPad mini to iOS 6.1 ... before that the iPads were seen in iTunes but I don't recall seeing them since.
Connected the one iPad that is left offline and so is still only on iOS 6.0.1 ... connected via USB and it appeared in iTunes! In addition, the iOS 6.1 iPads also suddenly appeared via WiFi! Although the mother-in-law's iPad still on iOS 6.0.1 is still missing via WiFi, but no matter.
So something related to the iPad mini being updated to iOS 6.1 in addition to iTunes 11.0.1 ... ?!
Anyway, all the iPads are now showing up in Devices in iTunes since I connected the old iPad ... strange.