I'm in exactly the same boat. I'm seriously considering upgrading from my 2007 MacBook, but would have a decrease in disk space as a result. Everything the Air does meets my demands but for the space. To tether an external drive would go against the idea of an ultraportable machine for me.
Do you have to have all of those songs with you at one time? I understand what you're saying but I guess... hmm.... you don't have a MP3 player or an iPod or something? that would allow you to load the library off of the external drive, put what you want to listen on that, then hear your music while also being able to save space.
I think that if you're set on the machine, you can manage with the storage; heck get a small, usb powered external HDD and keep it in your pants pocket or right next to the machine, they aren't larger than your standard wallet now anyway. 320gb ones are around 50 bucks.