I've just spent way too many hours trying to get my Asus eeePC to print to the Canon MP530 printer connected to thee USB port on my Apple Airport Express.
I could post links to all the suggestions I tried which didn't work and impress you with stories of my tenacity-- but chances are, if you've found your way here, you are looking for a solution and have wasted enough time already.
Here's what I did:
1) unplug Canon's USB cable from the Airport Express and plug it in to the USB port on your eeePC.
2) Add a printer, but make sure you use the local printer option. Your eeePC should automatically detect the printer on your USB port, as well as a FAX port.
3) Choose the USB port and continue. The wizard will automatically figure out the driver(s) to use. Be sure to print a test page-- you will need the info on the test page for later steps.
4) After the test page prints successfully, disconnect the USB cable from the eeePC and put it back into the Airport Express.
5) Add another printer on the eeePC, but this time choose a network printer. Don't use Windows or Unix; select "Other" and then use AppSocket/HPJetDirect port.
6) Here comes the potentially tricky part: you need to figure out which TCP/IP port your Airport Express is using for printer communication. It's supposed to be 9100, but for some reason mine is using 9101. I found using the "Find" button in combination with the little port scanner tool to be very effective in determining which port to use.
7) Once you've pinned down the port info, you're going to need to specify the printer model and driver. This is where the printer test page comes in. Use the
information on the Model: line to figure out the printer model and driver to pick.
For example, I used the Canon Multipass C2500, with Foomatic + bjc600.
8) Print a test page to confirm everything is working right.
Trust me, this way is much easier than some of the crazy stuff I was seeing on the forums.