Sync will resume when <x> is available - no Sync summary checkbox

iTunes 10.5, IOS 5, Mac hosts (one mini, one Macpro.)

Symptoms:

1) In the Options pane of the Summary tab for my iPad, there is no option "Sync with this iPad over wifi.

2) When the iPad is removed from the USB port, the iPad disappears immediately from the iTunes device list

I have checked:

o every forum I could think of

o Apple support docs: http://support.apple.com/kb/TS4062

I have tried:

o rebooting the router

o restarting the computer

o restarting the iPad

o hooking the computer to the same wifi network (normally its on ethernet, not wifi)

Manual sync works. Backup to the cloud for data works. But there's no wifi sync.

This problem is occurring with two different iPads and two different host computers (both Macs). As near as I can tell, everything is up to date and meets apple's requirements as listed at http://support.apple.com/kb/TS4062

Can anyone help?