The Phobos makefile (posix.mak) does not specify the architecture for building druntime, which defaults to 32bit even on if Phobos is being compiled for 64bit, the result is a broken lib.
My fixed Makefile passes the MODEL=64 parameter down to druntime's make process, resulting in nice and working builds on both, 32 and 64bit systems. I will post it in just a sec.
Happy hacking on 64 bit systems :P
- Sascha Heylik