... i think your idea that your limited to 15 partitions is erroneous.
I have been living under the same impression as the original poster. The following is from the pdisk manual entry:
Creating more than fifteen partitions is not advised.
There is currently a bug in the some (all?) of the kernels
which causes access to the whole disk fail if more than
fifteen partitions are in the map.
I am sure the debian mac-fdisk man page contains the same caveat
(certainly we have been advised on this list that they are versions of one
and the same program).
Granted, the above reference to "the kernel" is ambiguous; in likelyhood the author means mach rather than linux. For safety's sake, however, I am inclined to respect the suggested limit. Probably anyone multi-booting with MacOSX or other BSD-derivative would be advised to do the same until this situation is better clarified.
-R.