The Linksys GPL source code at ftp://ftp-eng.cisco.com/pub/opensource/linksys/ or ftp://ftp-eng.cisco.com/pub/opensource/smallbusiness/
The version 2.00.05 source code builds on Ubuntu 9.10, using the prebuilt toolchain.
It builds a somewhat-bootable "cybertan_rom_bin" image, but that built image is not the same as the image in the firmware upgrade binary.
It also builds the u-boot binary, including the secret console-locking code.
What seems to be missing is means to build the "cybertan_half_bin" image, and the tool to turn those images into a flashable binary (with the "WRP400 FiRmWaRe" header).
From examination of the source code, it seems that the WRP400 is just a rebranded version of the CyberTan WGV414 (http://www.cybertan.com.tw/Products/WGV414.htm).
The u-boot configuration in the source code is "cywgv414dls".