I've seen the theory posed on several websites that Dumbledore had the "gleam" because he's really evil and happy to know that one layer of Harry's protection had been removed.
Um, sorry folks, but--aside from the fact that Jo would be instilling a horrible fear of headmasters in all of her young readers--do you really think that Harry would still be alive if Dumbledore was evil and wanted him dead?
Aside from the fact that he could have thrown him in a river when he was a baby, how many times has Dumbledore been alone with Harry? How many times has he stood between Harry and danger? How many times has he saved Harry's life? And how 'bout the fact that he basically, um, died to protect Harry on top of the astronomy tower?
(Ok, so I get a little touchy when people start impuning bad motives to my Dumbledore. Human he is--I'm perfectly willing to admit that. But evil? Those are fighting words!)