The Raspberry Pi Foundation announced on its blog that the first batch if the Raspberry Pis is facing a small delay because the manufacturer used RJ45 connectors without integrated pulse transformers:
As you’ll have noticed, there’s been a bit of a delay in shipping the first batch of Raspberry Pis out to people. This is because of a hardware parts substitution that was made in the factory by accident: specifically, where we’d specified jacks with integrated magnetics in the BOM and schematics, the factory soldered in non-magnetic jacks. No magnetics means no network connection. We’ve known about this for four days now, but we haven’t been able to tell you about it because it meant we had to do some further tests to make sure that nothing else was affected.