After witnessing the Red Sox come back from down 0-3 to beat the Yankees in 2004 (and then go on to win the World Series for the first time in 86 years) I've become a big believer in the "it ain't over til it's over" adage. I've also seen some of my teams blow commanding leads in series.
The Lakers' backs are against the wall, for sure... especially going to Dallas for Games 3 & 4. But it ain't over yet. Especially with their experience and talent level.
Besides, down 0-2 is certainly not nearly as bad as being down 0-3. Game 3 for both the Heat/Celtics and Lakers/Mavs will be pivot games. If the Celtics and Lakers can win Game 3, they're right back in it. If not, they're both pretty well fooked.