The Airport Express:
1) You DO NOT need to have an existing Airport Extreme Base Station.
1.5) It will connect 802.11b (Old Airport + PCs with Cards) and Macs with Airport Extreme cards (802.11g).
2) Think of the Airport Express as a Airport Extreme, with no modem or external antenna option.
3) This means, you can create a completely new network for up to ten wireless clients (more on this later) which can connect to the internet via the WAN port.
4) However, if you already have an Extreme base station and network this will act a repeater, extending your existing network's range.
5) The Express has a WAN port which means you can connect you ethernet DSL/Cable router directly to it. Using normal ethernet settings (be it DHCP or Manually setup - you will know this) the wireless computers will then connect to the Internet via the Express.
6) If you have an Extreme base station with an internet connection, then the Express will just act as an extender. If for example your PowerBook is in range of the Express, the signal will be bounced to your Extreme which will connect to the internet.
6b) Not sure if this applies to shared Dialup connections but if you have the 'Connect automatically...' setting on your airport, I don't see why it wouldn't.
7) There is a USB port on the Express. This means you can share your printer wirelessly!
7b) If you have an Extreme base station, then Rendezvous will more than likely play its part in sharing the printer connected to either Extreme or Express. This means for me, I no longer have to have my printer on my desk next to my G5 and airport extreme station. I can have an Express in another room with the printer and next to... my stereo...
8) So we come to AirTunes. The Airport Express has what many of us have been waiting for - an audio out port.
9) This port can connect to any hifi / stereo / set of speaks. Yes, you can have JUST speakers if you want, connected to your Express, say, in your Living Room.
9b) There is a connection kit available from Apple at extra charge. This allows you to connect to an Optical IN, RCA plugs (AV plugs in EU, UK).
10) For it to work you MUST download iTunes 4.6, which came out today.
11) The Express automatically detects you having speakers connected, which sends a little message to iTunes telling it there are external speakers.
11b) Then say you want to play 'Marilyn Manson' so the whole Living room can hear (your mum's will be happy) - you just click 'Use Airport Express' box in iTunes - very easy.
Hope this helps... i'll add more if people ask, and if i've missed things.