Well to be explicit, the cannot netboot directly. I've built a u-boot that boots from the network via NFS on other arm based systems. You still need an MMC card to start it all off but once you get the network boot loader loaded and running you can do what ever you want. I started with some code for the Pandaboard that used that trick to boot from a USB attached drive.
The amazing thing about this project? Someone managed to get 64 RasPi's all in the same place!