It seems that when upgrading to Lion you lose the ability to "disable" IPv6. After upgrading to Lion your IPv6 config will still be set to "off", but if you change it, that choice goes away. You are only left with Link-Local, Automatic, and Manual as choices. None of these will allow you to connect to an Airport Express.
According to this post on the Apple forums, setting it to "Link-Local" will disable it. However your iTunes doesn't think so. So following these instruction from that same post you can return it to the "off" setting that allows iTunes to work properly with your Airport Express.
I had to change both the Airport and the Ethernet configs to __INACTIVE__ before it would allow me to connect to my Airport Express.
Here is what worked for me in Lion.
I edited /Library/Preferences/SystemConfiguration/preferences.plist
and changed the IPv6 ConfigMethod's string to __INACTIVE__ manually for the appropriate adapter. This is what I believe the GUI would change in SL10.6. Looks like it still accepts the option, Just dosen't provied it.
sudo vi /Library/Preferences/SystemConfiguration/preferences.plist
The section under your unique adapter key will look like this when you're done.
When completed reboot. Your inet6 address should not show for that adapter in ifconfig anymore.
Hope this helps.