I just successfully replaced my iPhone 3GS battery today, as mentioned above, make sure you slide the sim card tray back in as soon as the slot is aligned correctly, it'll help hold the logic board down and in the correct position. I also had to apply a larger than expected amount of force to the logic board in order to reattach the side screws.
The camera was one of the hardest steps to replace, I also dropped the small metal clip for the camera, but it's easily reattached. Realigning the camera was easier for me when most of the other internal screws were already attached.
I did notice that once I'd finished the reassembly, my phone complained about invalid certificates for email constantly. This ended up being resolved about 2 minutes after my phone booted, due to the date & time being reset!! I had quite the panic at first, I hope this saves someone else the brain trauma :)
Overall, an AWESOME guide.