I initially saw this at this post on TUAW, which links through to the original instructions from Mike Matas. Here’s a picture of the life poster from Mike’s blog:
Essentially you take 98 individual pictures and turn them into a 30”x20” poster. Full instructions are on Mike’s blog and you should definitely read them because it will make the project very easy. Mike estimates the cost at about $29 per poster.
I really don’t see any reason why you couldn’t pull off the same sort of project with Flickr if you don’t have/use iPhoto.