Yesterday, I praised the release of Apple’s Airport Express. I’ve done a little more reading on their site, and am somewhat less enthusiastic. A higher level Apple nerd can correct me, but it looks like the thing will only work with Apple’s base station.
If you already have a wireless network in your home and would like to extend its range, AirPort Express is your answer. Suppose you want to connect to the Internet with your PowerBook in an area that lies beyond the 150-foot range of your AirPort Express or AirPort Extreme Base Station. You can use AirPort Express as a wireless bridge to extend the range of your primary base station.(5)
(5) AirPort Extreme and AirPort Express can extend the range only of an AirPort Extreme or AirPort Express wireless network.
It’s a pretty cool device, but I’m not going to shell out 300 (Canadian) bones for the Extreme plus $150 for the Airport Express. Especially when I just bought a totally-adequate 80211g wireless hub for all of $100.
DarrenBarefoot.com 
That was my reaction too, but apparently, it may work with non Apple base stations as well.
http://www.gizmodo.com/archives/why-apples-airport-express-may-unofficially-extend-nonairport-networks-015834.php
I certainly hope it does, because I’m in the same situation. I imagine the “* Subject to regulatory approval prior to release in Canada” should give us lots of time to find out how well it works before we can actually buy one…
I was also concerned about this, and was reading through their smallprint. It seems to me that if you want the range-extending functionality, you need your other equipment to be 802.11g (though maybe not only Apple’s); but if you just want the Airtunes feature, 802.11b will work. I won’t be rushing to the Apple Store until there’s more info on the various interop issues, though
Why they don’t have simple repeaters like HAMs do I don’t know. The repeater shouldn’t need any knowledge at all, only what channel the hub runs on.