I generally don't hide that I like to game. However, I do generally leave out that I play World of Warcraft until the person gets to know me better.
I do so, as you can probably guess, because people pre-judge you as some basement dwelling, mouth breathing nerd...when in reality this is rarely the case with most WoW players (at least I've found).
It's funny though, because I've had people do the same thing with me. For example - I was working as an RA while I was in college, and one of my bosses found out that I played WoW (If I recall correctly he stopped by while I was raiding one weekend). I then learned that he played WoW, and was a pretty hardcore raider, and was on the same server as me on the opposite faction. I then camped BRS on my rogue, and ganked his gnome ass...