I just did this.
On the iOS device, in settings-icloud, turn notes on.
On the Mac, in preferences-iCloud, check Mail & Notes
On the Mac, in Mail-preferences-composing-create notes in: iCloud
On the Mac, in Mail, you should see an "on my mac" account and another for iCloud under notes.
Sync the iOS device to the Mac. In iTunes on the device-info window, select sync notes. Then apply the change and sync. All notes on the device should appear in the iCloud notes folder in Mail as well as on the iCloud notes tab. Then deselect the sync square atain on the Mac in iTunes on the device-info window, tell it to remove notes when it asks, then apply the change.
On the Mac, drag notes that are in the "on my mac" account to the iCloud account.
Repeat for all iOS devices.
All notes should now appear on all iOS devices and all Macs configured for your iCloud account.
Bottom line is that you can't do it on Mountain Lion anymore, iTunes in Mountain Lion no longer sync Notes between "On My iPhone (iPad)" and "On My Mac".
If you have access to another Mac with Snow Loepard or Lion, just create a new account, and sync your iPhone's Notes (I only sync Notes and unchecked all other sync options, its very fast once backup is done), and move all the "On My Mac" notes to iCloud or any other IMAP account. I didn't have access to another Mac at that time, so I just install Parallel Demo, and install Lion on the virtual machine.
Hope this helps.