Nick, my dad has a '99 Siera (5.3 liter) and it has the previous variation of the oil change warning. It will pop up when it's time to change the oil. My dad has kept track of the mileage and the light has been fairly consistant to pop on every 7-10k miles. This is in the range of what the owners manual recommends. Some times it'll come on a little early, sometimes a little late. He hasn't anylized all his driving to see what makes the intervals change, but he has never had a problem with the truck. It's actually nearly 150k miles and hasn't had a single issue.

I'm with Chris, if GM will bet their ass to a 15k miles oil change, I wouldn't expect it to be bad. Can you imagine what kind of money they'd lose if they had engine failures under warranty due to the long interval? I'd say you're golden.