On 3.0.4 (and probably all other recent kernel versions), HIGHMEM memory (HIGHMEM64G in my tests) is apparently ignored during memtest.
Looking at arch/x86/mm/memtest.c, no special mapping is performed (kmap/kunmap?), so it seems that at most ~895 MiB can be tested in 32-bit x86 kernels.
This might not appear like an important issue (as there are other memory testing tools available), but memtest is extremely useful for anti-forensic memory wiping on shutdown/reboot in security-oriented distributions like Liberté Linux and Tails, and there is no other good substitute.
See, for instance, some background in Debian bug
I tried posting to the linux-mm mailing list, but the mails were ignored.
If you want it fixed I think you will need to write the code and submit it. Nobody else seems to be concerned about it.