Achievement Unlocked, June 16th 2009, 8:00 a.m.(ish). Thank you to everyone involved in the production of this guide for your help I was beginning to wonder if I'd ever get this one.
Funny thing is, I got every powerup on every level and still didn't have the achievement. Went through each level on all gametypes, hit start at the beginning and they all said "All powerups found!". Turns out I had all powerups from Arsenal before the latest patch, so when I re-played that one just this morning (and picked up all powerups/vials/etc) again, it worked and I finally got the achievement.
Thanks again and much kudos!