I use the following method to update rom/gapps/kernel, so far no system r/w problems found!
1 reboot to CWM v184.108.40.206
2 apply cm7kang-O2x-RC1-89-PDroid.zip
3 apply gapps-gb-20120406-237-signed.zip (if needed)
4 apply 1204151346-ETaNa_OC_OTF-3...
I have the same problem on the /system r/w option as anto986.
The rw mode on /system is only available once, then the second time it doesn't work anymore.
I even tried to mount /system in recovery via ADB and I've got a "Segmentation fault".