I asked myself this exact question today. Locating my apache httpd binary and running:

./httpd -v

Server version: Apache/2.2.8 (Unix)
Server built: Feb 20 2008 12:01:56

Told me I had Apache 2.2.8 which was cool but I wanted to know what aracheture is was, 32-bit or 64-bit. A little playing round lead me to find out the following way to work out the aracheture of my apache install.

Locate your apache httpd binary and run:

./apchectl -V

Server version: Apache/2.2.8 (Unix)
Server built: Feb 20 2008 12:01:56
Server's Module Magic Number: 20051115:11
Server loaded: APR 1.2.7, APR-Util 1.2.7
Compiled using: APR 1.2.7, APR-Util 1.2.7
Architecture: 32-bit
Server MPM: Prefork
threaded: no
forked: yes (variable process count)
Server compiled with....
-D APACHE_MPM_DIR="server/mpm/prefork"
-D APR_HAS_SENDFILE
-D APR_HAS_MMAP
-D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
-D APR_USE_FLOCK_SERIALIZE
-D APR_USE_PTHREAD_SERIALIZE
-D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
-D APR_HAS_OTHER_CHILD
-D AP_HAVE_RELIABLE_PIPED_LOGS
-D DYNAMIC_MODULE_LIMIT=128
-D HTTPD_ROOT="/usr"
-D SUEXEC_BIN="/usr/bin/suexec"
-D DEFAULT_PIDLOG="/private/var/run/httpd.pid"
-D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
-D DEFAULT_LOCKFILE="/private/var/run/accept.lock"
-D DEFAULT_ERRORLOG="logs/error_log"
-D AP_TYPES_CONFIG_FILE="/private/etc/apache2/mime.types"
-D SERVER_CONFIG_FILE="/private/etc/apache2/httpd.conf"