I have one box running two monitors with Windows 7 and PCLinuxOS 2010 Beta2. It used to spread out the desktop just fine with PCLinuxOS until I updated to the beta version. Now the ATI graphics card driver can only handle the two monitors without the spread (i.e., identical desktops on each monitor). In the past, I've had to resort to downloading and installing the driver from ATI/AMD to get the xinerama spread-out; for some reason, the PCLinuxOS drivers for this ATI 9600 card have always been problematic for me in this regard. If it doesn't get sorted by the time the final version arrives, I'll probably give the OEM driver another go. That fallback has always worked for me so far.
I did notice somewhere else in the forum that Texstar is messing with the xorg configuration for the ATI cards, BTW.