If BBMASK is set in e.g. a distro configuration file and you want to append/prepend to this from local.conf using BBMASK_append/prepend this has currently no effect since the append/prepend happens after the value of BBMASK is used. Inspecting the value with "bitbake -e <something> | grep ^BBMASK" will show the value with the append/prepend, but if BBMASK is printed in cooker.py where it is used the original value is used.
I believe that with the recent data store changes, this will now no longer be an issue (as of http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=a730981b75ebb528e447eb3e41c83440dbc133bd).