Ok, I gave up the "you never need to reset anything, it just works" -thing. And yes, for the first time ever since zillion iOS upgrades I did reset the network settings. I lost maybe 30 wireless hotspot passwords, but at least client certificates still exist which makes the reset much more tolerable thing. Actually the upgrade path worked overall surprisingly well until these days, considering, I started with iOS4? or even with iOS3 and went all the way to iOS6.1 while also gradually migrating the old setup via backup/restore to new devices and new iOS versions - although maybe it did help to have old and new device always in the same iOS version during the restore from backup.
I tested again Personal Hotspot hosting it with iPad and iPhone 4s via wifi - all clients (iPhone 3s/iOS4.x, iPhone 4 (iOS6.1), iPhone 4s (iOS6.1), iPad 3 (iOS 6.1) again connect very fast without any problems trying with different combinations. Ok course.. I don't know if this is permanent fix, nor if the restart of the devices fixed the issue, like earlier today, or if resetting network settings helped. We'll see along the coming days.
As another quick thing - the iPhone 4 ambient light sensor DOES work still. The changes are just much more subtle compared to iPad/iPhone 4s and the brightness slider doesn't move either in iPhone 4 like with newer devices same way. However I can see definitely difference when unlocking the phone next to light source vs in dark room.
Now, if only the mobiles would stay alive in cold weather without warming them with lighter every now and then I might be actually eehh.. happy? 🙂
-p