Re:-1 flamebait (Score 4, Informative)

While theoretically correct, the practice is different: everyone is allowed to collaborate when the software has a BSD license. However, since it is not mandatory to publish the code, it really doesn't matter that much.

For example, the ASUS WL-500g (Linksys like router with USB port) its firmware is recompilable and hackable by you and me since it is (mainly) GPLed code. The newer SL1000/SL5000 (vpn routers) contain several BSD modules which ruin the party:

[From: http://website.wl500g.info/beta/firmware.php?fid=3 3 ]

Changelog:
SL1000 and SL500 GPL source code
Before using the source code, please note:

1. The router's firewall and VPN are licensed 3rd party code and are not subjected to GPL terms.
2. Several software modules are derived from BSD codes, which ASUS won't release. ...and therefore:

[From: http://wl500g.info/showthread.php?t=3417 ]
There are no chance to build something useful from this sources.