In 2005 we could created "Life Poster" by Print Menu in iPhoto 5 (iLife '05 Suite). But now we cannot do it in iPhoto 6 and current version iPhoto 7.
After surf in Google! I found workflow about Automator, created by Jim Heid from macilife.com. Unlucky, his script are outdate version of Automator in Leopard.
From that script, I inspire to select new update script inside Automator in Mac OS X 10.5.
3 Steps for "Life Poster" with Automator in Leopard.
Step 1: Drop photos (72 pieces - in horizontal) in Automator - 12 rows and 6 columns.
Step 2: If your photos are big size, recommend to resize maximum to 2048 pixel (by dimension).
Step 3: Choose "Create Thumbnail Poster", click Run and wait a minute.
You can get output file (.PDF) to print in poster at print shop near you.
Tip: You can select thumbnails option for change display thumbnails in Square, Horizontal or Vertical, and do not mix horizontal and vertical photos.
Download LifePoster_Leopard workflow here.