Despite my earlier voiced contempt for MacPorts, I present you build_macports64, a small script which bootstraps a 64-bit MacPorts environment from their SVN repository. No manual intervention is required.
Dependencies: 32-bit MacPorts installed (you want to keep it anyway for the moment), and a configured sudo. By default, it installs to /opt/local64/.
I have the following packages build and installed (here, universal means x86_64):
% port installed
The following ports are currently installed:
apr @1.3.3_1+universal (active)
apr-util @1.3.4_1+universal (active)
bzip2 @1.0.5_2+universal (active)
curl @7.19.5_0+universal (active)
db45 @4.5.20_4+universal (active)
db46 @4.6.21_5+universal (active)
expat @2.0.1_0+universal (active)
gettext @0.17_4+universal (active)
gmake @3.81_0+universal (active)
gperf @3.0.4_0+universal (active)
libiconv @1.12_2+universal (active)
ncurses @5.7_0+universal (active)
ncursesw @5.7_0+universal (active)
openssl @0.9.8k_0+universal (active)
pkgconfig @0.23_1+universal (active)
popt @1.15_0+universal (active)
readline @6.0.000_1+universal (active)
sqlite3 @3.6.14.1_0+universal (active)
tcl @8.5.6_0+threads+universal (active)
zlib @1.2.3_2+universal (active)
Sadly, at the moment there are some crucial packages which do not
build as x86_64 even if requested (perl, boost). For others,
the build just breaks, e.g., because CFLAGS are
not passed properly. But that was to be expected, I guess. With
more people using it and reporting/fixing bugs, the situation is
likely to improve over time.