Archive vs. Delete in iPhone Mail?

I've heard a lot (well, okay, a little) talk about how Gmail users can configure Mail under iOS 4.0 to archive messages rather than deleting them, yet I can't figure out how to set this up. I've been through the prefs pretty thoroughly, and I'm stumped. I may just trash and re-add my Gmail account, but I thought I'd throw it out there, to see if anybody has found a solution. Thanks in advance if you have!

Evan