Everything was good with my dual 5850 until I added 4 more. Ubuntu Natty simply stuck at a blank screen (without internet connection) after the logo "Ubuntu" is loaded. I tried the following (through recovery mode):

1. Reinstall the driver: tried auto install from Ubuntu, and from the package downloaded from ATI website. Also tried manual install.
2. aticonfig --initial -f --adapter=all
3. aticonfig --initial -f --adapter=0,1,2,3,4,5
4. In /etc/X11/xorg.conf ServerLayout section, change "RightOf ...screen...1", "RightOf ...screen...2..." .... to "RightOf ...screen...0..."
5. In /etc/X11/xorg.conf ServerLayout section, change "RightOf..." to 0 0

None of them work, I couldn't enter login screen unless I run "aticonfig --initial -f" which only enables the first device. It doesn't solve the problem.
Of course, all cards can be detected by aticonfig (as seen by aticonfig --list-adapters), and atiflash in MS-DOS.

In windows I have to trick the OS by using some dummy DVI adapters - that's why I don't want to switch to Windows.