For my PC (Windows XP Professional) I did the following in order to get the C4400 to work with my Airport Express
Start by using the software CD to load the drivers for the printer
Make sure you plug the printer directly into your PC when the software tells you (not before) and let the program finish its work. You will have to go through the process of printing a sample page, scan it, etc. So be patient.
After you are done - unplug the printer and plug it back into your airport express, Launch the Bonjour Printer Wizard, follow the steps and make sure you select the HP C4400. **Also set the printer as your default** Once that has been completed go to your start menu and launch the Printers & Faxes menu. The status for your C4400 printer should show as "ready" and under the location column the name of your airport express should be listed as well. If you see those two things try to print from your computer. If you see the printer listed more than once - you must delete all of them and start again. I printed a three page document and it printed all, in addition to clearing the que.
The HP Digital Imaging Monitor will show that your printer is disconnected, but it appears that you will only need that when you are scanning, making copies or using the software to manipulate photos. (if anyone knows how to fix that - tell me) 🙂
Good luck!
Lastly, I haven't received my MacBook Pro yet, but if anyone knows how to set that up to work on the Airport Express - I would gratefully appreciate the instructions!