Pg. 278
"Rather than roll damage, determine the maximum damage you can roll with your attack. This is your critical damage."
In a 3e mentality, I want to say sneak attacks aren't maximized, but the text implies that it's the maximum for the "Attack" not the weapon. FWIW, the next paragraph specifically mentions that extra damage from high crit weapons and magical properties is not maximized. Sneak attack is not mentioned.
The entry for sneak attack makes no mention of critical hits on Pg. 117, and the index only lists two places for critical hits. The other location (pg 276) is far less specific, and doesn't mention extra damage at all.
I think it looks pretty clear that SA is maximized. (And the Warlock's curse and the Ranger's quarry, at that.)