1. It's a new product and a new team. Delays are a normal part of any ambitious project and don't necessarily mean the product won't ship.
2. For instance, consider that forecasted sales determine forecasted timelines
Their original shipping dates are based on raising a projected $100,000. At $99 for the cheapest Pebble, one can imagine that their worst case scenario was to ship roughly 1,000 Pebbles within their original timelines. Their Kickstarter actually raised over $10.25 million and consequently they are supposed to ship 85x more Pebbles than that. It's not hard to understand that such a dramatic difference in actuals vs. forecasts will result in a dramatic change in shipping time frames.
3. More importantly, constant communication from the Pebble team shows they are making steady progress (current backers have until the end of the month to make their colour selections, and in the past month their team took advantage of their 70k backers to perform free market research into desired Pebble colours, which is a wise business decision); I really don't see much to objectively worry about.
4. Backers of the project weren't ordering anything, and are naive if they think they were. Anybody know the Latin for "backer beware"?