It looks my earlier findings were a bit premature. I’ve been given access to a Core2Duo Mac mini by Ceriel of Vakantieland.nl and Crashplan.nl to test the findings I had when compiling the MAMP stack in 64 bits. There are some minor differences in compiling MySQL and PHP as the earlier solution does not work for them.

Apache:
CFLAGS="-arch x86_64" \
./configure --enable-layout=DIYMacServer \
  --enable-mods-shared=all \
  --with-ssl=/usr \
  --with-mpm=prefork \
  --disable-unique-id \
  --enable-ssl \
  --enable-dav \
  --enable-cache \
  --enable-proxy \
  --enable-logio \
  --enable-deflate \
  --with-included-apr \
  --enable-cgi \
  --enable-cgid \
  --enable-suexec

MySQL:
CFLAGS="-arch x86_64" \
CXXFLAGS="-arch x86_64" \
./configure --prefix=/usr/local/mysql \
  --localstatedir=/usr/local/mysql/data \
  --libexecdir=/usr/local/mysql/bin \
  --libdir=/usr/local/mysql/lib \
  --with-server-suffix=-standard \
  --enable-thread-safe-client \
  --enable-local-infile \
  --enable-shared \
  --with-zlib-dir=bundled \
  --with-big-tables \
  --with-readline \
  --with-archive-storage-engine \
  --with-innodb \
  --without-docs \
  --without-bench

PHP:
CFLAGS="-arch x86_64" \
CXXFLAGS="-arch x86_64" \
./configure --prefix=/usr/local/php5 \
--mandir=/usr/share/man \
--infodir=/usr/share/info \
--sysconfdir=/etc \
--with-zlib \
--with-zlib-dir=/usr \
--with-openssl \
--without-iconv \
--enable-exif \
--enable-ftp \
--enable-mbstring \
--enable-mbregex \
--enable-sockets \
--with-mysql=/usr/local/mysql \
--with-mysqli=/usr/local/mysql/bin/mysql_config \
--with-apxs2=/usr/local/apache2/bin/apxs

In Leopard there is a problem with the iconv library, somehow that is not 64 bits or something like that. When compiling 32 bits it works. I guess if you really want iconv you will have to recompile the inconv library to get that one working. Currently I’m working to get IMAP working for postfixadmin.