TNT finally delivered my new AirPort Express today.
No, it's not the same thing as AirPort Extreme or the other basestation models. It's basically a WLAN-to-Audio bridge that you can use e.g. for streaming music to your bedroom, which is exactly what I use it for.
To be fair, AirPort Express can do more than that. It's also a USB-to-WLAN printer adapter, and in fact, it does function as a pocket-size WLAN access point / gateway if you want it to.
Basically what Apple did was they put four interfaces in it; audio, USB, Ethernet and WLAN, and let you choose what to do with them. It can create it's own WLAN network or connect to an existing one, which is pretty cool... I'll be bringing this little box on my trips abroad to check if it works with the wireless networks offered by business hotels.
So far the device seems to be working perfectly. The only configuration it needed was an initial setup to choose the WLAN network name and WEP key. After that, the LED went green and iTunes recognized a new audio output. And it works in both Mac OS X and Windows iTunes!
The only problem I ran into was my network, which consists of two separate subnets for wired and wireless clients. Apparently, Apple's implementation of Rendezvous, which iTunes uses to talk to AirPort Extreme, can only handle devices within the same subnet. I'm hoping that this will be somehow fixed by Apple, or that I'll be able to add a multicast router/bridge to my Debian box to enable the Rendezvous traffic between the networks.