I have done few testing. You are correct. I think my eyes have lied to me.
iPhone 4 with iOS 5.0 in sleep mode - Cellular data off (airplane mode)
- time out when I tried to ping
- had connection when i pressed the home button to wake it up (but it took very quickly to reconnect)
iPhone 5 with iOS 6.0 in sleep mode - Cellular data off (airplane mode)
- time out when I tried to ping
- had connection when i pressed the home button to wake it up (but it took a bit longer time to reconnect compare to iPhone 4) - I am on WPA2 Enterprise wifi connection
my conclusion:
I think my iPhone 4 has been disconnecting the wifi connection in sleep mode, but it gets reconnect so quick once I press the home button, so I did not realize it was disconnected.
2nd thing I have noticed is:
when iPhone 5 in sleep mode and in airplane mode, I tried to send an iMessages from my iPhone 4, it surely got deliveried to iPhone5 and it woke it up. That is interesting. If the wifi was disconnected, how could I get the iMessages in iPhone 5? does the iOS connect to wifi automatically every now and then to check if there is any push? I do notice that there was a delay to delivery the message to iPhone 5 while it was in sleep mode, but it did delivery thou.