You require 10 of the 30 tags to obtain this achievement. The locations for all the cog tags are as follows: (refer to the video for exact locations (not my video!!!))
Act 1 Chapter 1, 14 years after E-Day, #1, #2, #3
Act 1 Chapter 2, Trial by Fire, #4, #5
Act 1 Chapter 3, Fish in a Barrel, Tag, #6
Act 1 Chapter 5, Knock Knock, Tags, #7
Act 1 Chapter 6, Hammer, Tags, #8, #9
Act 1 Chapter 8, China Shop, #10, #11, #12
Act 2 Chapter 1, Tick Tick Boom, #13,
Act 2 Chapter 2, Grist, #13, #14, #15
Act 2 Chapter 4, Lethal Dusk, #16, #17
Act 2 Chapter 5, Dark Labyrinth, #18
Act 3 Chapter 1, Downpour, #19
Act 3 Chapter 2, Evolution, #20
Act 3 Chapter 3, Coalition Cargo, #21
Act 3 Chapter 4, Darkest Before Dawn, #22, #23, #24
Act 3 Chapter 5, Angry Titan, #24,
Act 4 Chapter 1, Campus Grinder, #25
Act 4 Chapter 2, Bad to Worse, #26
Act 4 Chapter 5, Imaginary Place, #27
Act 5 Chapter 2, Train Wreck, #28, #29, #30
Edit 04/12/10: Added new video