I don't know if this happens on the PS360 side, but Nintendo forums have a lot of self-hating Nintendo fans, who say they grew up with Nintendo and that it's #1, but always bash the Wii. It's kinda funny.
Fanboyism generally though is based on feeling threatened. This is why you see people bashing games like Killzone 2 so heavily (becoz it looks better than most 360 games graphically) and why the same will happen to God Of Ware III. Even before Gears of War 2 came out, public reviews on metacritic gave the game like, a 2 out of 10 or some nonsense like that becoz of the Sony pre-emptive strike. Super Smash Bros. gets reviews of either a near-perfect "10" or an abysmal "0!". Resident Evil 5 is pretty much the EXACT SAME game on the PS3 and 360, yet there are already numerous articles asking which is better (especially graphically) and that is just ridiculous. Videogame journalists...YOU"RE NOT HELPING!!!.
I figure this time around Nintendo fans might not be as bad coz they consider themselves as "winning" right now. PS3 is dead last and trying to make up ground on 2nd place to they attack XBOX. And Microsoft is stuck in the middle, wanting to beat Nintendo but at the same time trying to keep Sony down. I think that's why I see most hate from 360'ers on the Wii side. Sony doesn't care enough to bash Nintendo at this point coz what's the use? If they frog-jump over Microsoft to make it to second place, they'll then change their target to include Nintendo also.
