High TechPlanet currently have a limited supply of the DR-900, and their asking price is €299 plus shipping. that converts to roughly $424, which might explain why it's not sold in the US.
Asus are in an experimental mood right now, and the DR-900 is one of the experiments. It's 1 of 3 ereader proposed by Asus (the third hasn't been released yet). The DR-900 is based around a 9" Sipix screen, and it has Wifi, Bluetooth, a microSD card slot, and according to some reports it also has a 3G connection.
Asus' other experiment in ebook readers is the eeeNote EA800, which also never made it to the US.