I've been researching this. It would be nice if something like this worked, since the price is right, but I don't think this will be fast enough to do what you want, since you will be using the ethernet drive as a wireless server for your music, rather than just streaming from one computer to the airport (like airtunes over an airport express). I considered hooking up a 500GB USB drive to the new Airport Extreme (basically the same as what you're proposing), but I decided against it after reading several reports on the internet of people having trouble using the Airport Extreme as an iTunes server. If you are thinking about hooking up the external drive directly to the laptop, make sure you take into account hard drive noise.
Please let me know how this works out. Although I haven't pulled the trigger yet, the best solution I've found so far for a 500GB music library (with built-in backup and room to expand to 2.25TB in RAID 5) is to buy an Infrant ReadyNAS NV+ and hard wire it to a giga-ethernet router (10/100/1000) in a media/LAN closet. It isn't a cheap solution, but it is very robust, fast and secure.
Please let me know how this works out. Although I haven't pulled the trigger yet, the best solution I've found so far for a 500GB music library (with built-in backup and room to expand to 2.25TB in RAID 5) is to buy an Infrant ReadyNAS NV+ and hard wire it to a giga-ethernet router (10/100/1000) in a media/LAN closet. It isn't a cheap solution, but it is very robust, fast and secure.