How the glass breaks is dependent on the glass, some just crack and hold its shape, others "explode." It depends on the force applied to the glass and type of glass.
As for the location of the chip, I dont understand why it wouldnt crack under pressure. Glass is not like acrylic where the corners are stronger than the open spaces. Glass is way more brittle. Inconsistent strength anywhere on the sheet of glass is a point of weakness. The silicone bonds add some strength to the corners, but if the glass were cracking under pressure, it would spread out towards the center of the tank.
@lorax: Could you explain more about why theres "no chance of the chip spreading"? I'm not a glass expert, but i have worked with glass and acrylic. In my experience, when pressure on glass is not evenly or gradually applied, ie a single point of excess pressure or weakness, the glass is more prone to cracking at that point <-- this is why aquascapers who use big rocks for their tanks put foam below their tank to even out and ease pressure at any one point. Its the same situation with car windshields, when theres a chip, cracks are much more likely to occur from the chipped spot.
There are other factors as well in terms of if the chip will affect the integrity of the tank, if the crack is near the top of the tank, theres less pressure on it. If its near the bottom of the tank, theres more pressure on it. Depth of the chip is also a big factor, a very shallow chip may stand up better to pressure, but if the chip is deep, ie half the thickness of the glass overall, then it may be cause for concern.
The best solution is to call petsmart and tell them about your situation and ask if they will exchange it.
When you say half filled, do you mean filled with gravel, decor, etc or just a test fill where its just water (always test fill tanks for leaks and cracks). If theres been no decor, draining it and wiping it down should be just fine, but with substrate or sumthing, you may have scratched the tank so it would be less likely for petsmart to accept a return.