If you watch the video explaining Pebble, it become pretty clear that the watch is, in essence, a new form factor for the iPhone. It’s smaller, it’s more use-case defined, but that’s what it is: A smaller mirror of your iPhone, strapped to you wrist. Pebble uses bluetooth connectivity to access the iPhone’s native capabilities, and then displays data, apps, and services on its high-resolution e-paper screen. It even has its own “app store” and (upcoming) SDK/API so people can write native apps to the device.
Yes, Pebble is useful but as pointed out by Gruber
Pebble is already hindered by the limited amount of interaction a Bluetooth device can obtain from an iPhone. E.g., it can notify you of incoming phone calls, but not messages or emails.
To add it can control music but Apple will only allow for so much via Bluetooth. You can't transfer files between iPhones or other phones for example by Bluetooth so how much functionality could this watch have with the iPhone? Almost extremely basic. The fact is, there is a product in place that when/if Apple is willing could kill the desire for a Pebble with an iPhone, the iPod Nano. Throw an NFC into it or Bluetooth, keep current form factor and switch over to iOS system. Eventually this will be possible but Apple is a bit busy, they need to get iCloud to be even better and more reliable as well as moderate offline access to it which it somewhat does though the main meal on Apple's plate is Siri which would, when combined with such an iPod Nano, if ever, well...just imagine.
Personally, the design just isn't all that appealing to me.