Check out the Newertech Mini Stack USB/Firewire HD with both USB and firewire hubs built in. It has the same footprint as the Mac Mini and the Airport Extreme:
http://eshop.macsales.com/shop/ministack/
I have one and it works great.
A couple of observations about the Airport Extreme though...
My External hard drive does not not seem to spin down when it is not in use. I thought a firmware update from Apple was supposed to fix this, but it didn't seem to make a difference to me. I don't believe this is specific to the Mini Stack.
Printing works great, but not on all printers. I have used several current Epson models and they work very well. Apple has another firmware issue though where the Airport loses the printer if the printer is turned off and back on again or disconnected. This is a pain because you have to reboot the router to get the printer back again.
If your printer is an all-in-one printer/scanner, keep in mind that you probably won't be able to scan through the Airport either. I know my Epson RX500 can't. For the few times I scan something, I pull the USB cable from the Mini Stack hub, and plug it into the Apple keyboard. Kind of a hassle, but it works. of course this also makes the Airport lose the printer, which is a real pain...
You milage may vary, but I am very happy with the Mini Stack - especially the way it looks on my desk. Now I just wish Apple would fix the problem with the Airport losing the printer...