Just found an article on LinuxDevices about a new NAS design from Broadcom. It uses the same CPU as the Asus WL-700g (BCM4780 SoC) but allows for three harddisks to be installed.
Read the full article here: http://www.linuxdevices.com/news/NS7290790007.html
While this is still a reference design, we will probably see some manufacturers picking this up which will result in some good contenders for the WL-700g. Too bad Broadcom didn't implement a hardware XOR engine. This will make RAID-5 unreasonably slow most likely. Besides that, the Broadcom CPU supports RAID levels 0, 1 and 10.