I don't know really if it's the same case but I have got some weird shadows too.
I attached a screen capture to this reply.
My platform is an ubuntu 11.04 with the free radeon driver on a R600 ati card.
OpenGL renderer string: Gallium 0.4 on AMD RV670
OpenGL version string: 2.1 Mesa 7.10.2
OpenGL shading language version string: 1.20
I use your lastest work with the ppa 0ad.dev
sudo add-apt-repository ppa:wfg/0ad.dev
There is a fix for the r300 driver but I don't know if the patch can be transfered to the r600g?
I can provide you more details if necessary.
Thanks for listening