I have an Airport Express and am using it to stream music from my devices to my stereo. I could get my iPhone to work, but never my MacBook. I finally dug in and did some Google-Fu this morning, and found this thread in the Apple support forum. Here are the relevant sections:
One option to try that seems to resolve iTunes streaming with OS X Lion is to disable IPv6. In previous versions of OS X, IPv6 was disabled by default. Not so in Lion and there is no option in the Network panel to do so.
To disable IPv6 for wireless, issue the following command: networksetup -setv6off Wi-Fi
To disable IPv6 for Ethernet, issue the following command: networksetup -setv6off Ethernet