Up until now the sparc64 kernel didn't use sparsemem thus some newer machines (notably UltraSPARC-IIIi based SMP) didn't use all of the available memory since each CPU has it's own memory controller and there were holes in the memory layout.
The following patch enables sparsemem for sparc64.
Created attachment 82796 [details, diff]
sparsemem for sparc64 2.6.16+
This was merged into 2.6.16-git2, likely going mainstream for 2.6.17.
Please include in gentoo-sources-2.6.16-*
fixed in gentoo-sources-2.6.16-r1 (genpatches-2.6.16-2)