the 2.6.11 series of kernels (possible 2.6.10) will break on certain LPAR'ed pSeries machines with out this patch http://linux.bkbits.net:8080/linux-2.5/cset@4248cb0a5sYVpmF0MKW6YM_snESiEA As you can see it has been accepted upstream, please add to patch list for 2.6.11 based gentoo kernels.
Created attachment 54869 [details, diff] iommu.patch Here is the patch which can be added
Fixed in gentoo-sources-2.6.11-r6