Let's be honest here for a second. Cracks don't appear. Cracks don't form. They don't develop. Your iPhone 3G cracked because you dropped it. 'Fess up! It's not a manufacturing defect if your phone takes a tumble down a flight of stairs. This is why we can't have nice things.
Just kidding. Apple
could have has another iPod nano or G4 Cube problem on its hands: careful users are seeing cracks and fissures on their new iPhone 3Gs. MacRumors has a thread about new, white iPhone 3Gs (which haven't been "dropped or pressed") that are beginning to show hairline cracks around the edges and near the buttons and headphone jack.
Reasonably, though, if you treat it like a phone, and use it every day, it's going to get worn. That's when cracks, scuffs, and abrasions happen. When you have a white finish on your iPhone, dirt will inevitably get in there, and even the most minute scratch will show up. Every phone I've ever owned I've dropped at some point, and it gets a scuff, or a scratch or a crack.
There are plenty of options to protect your sweetness, though: For example, there are all kinds of hard-shell cases you can buy. ZAGG's excellent invisibleSHIELD product is certainly durable, and could prevent dirt from getting into any fissures that appear on your iPhone's back cover. You can also wait a little while for the Golden Shellback, a vacuum-applied polymer that waterproofs any device, inside and out. Nifty.
Update: Commenters are literally pouring in to tell us that as careful as they've been with their iPhone 3Gs, even the most babyed devices are showing cracks. Joel Renda says, "The problem is not the plastic, but that the metal frame is too small for the plastic to lock on without causing the stress fractures." It's pretty clear Apple has a manufacturing defect on its hands.
For those with cracks, taking it back to the Apple Store (or possibly the mobile phone retailer where you made your original purchase) is your only recourse. Several people have noted here and elsewhere that they've successfully had their handset replaced after a careful inspection.