Like NASCAR, Country Music and StarWars, gamers are enthusiasts but for an entire category not simply a particular game or brand. Yes, there are Halo 2 fans/experts (and XBOX 360). But gamers are a proud and active bunch about all things 'games'. Coincidentally they are tech savvy and well-connected on the Web. This makes them an interesting case for measurement. Gamespot is combining their own Web data (Trax) including user data about what they are most looking forward to with NPD Funworld's stats which track sales. This will be announced today at E3.
Connecting online activity to sales (and then effecting that activity/sales) is the type of measurement clients long for.