Hi, I think I might have found a person who had modded their gamerscore. I won't give out their name, but I will tell you why I think he has been modding.
On Halo: Reach, he has achievements unlocked in a completely random order, umocking the Legendary achievements after finishing half of the missions, then the Normal and Heroic achievements after a load of other achievement. Which makes no sense. Now, this wouldn't be so suspicious, if he hadn't unlocked all of his DLC achievements while offline. The same goes for Halo 3, all earned offline. And in Assassins Creed 2, he unlocked "Show Your Colours", then a few random achievements, then 'In Memory of Petruicco".
I filed a complaint against him, but wasn't sure exactly what to put for the reason. I put 'System Tampering', however it's been 24 hours and nothing has happened. So, either he's smehow legit, or I picked the wrong option. :S