According to IGN, the next generation of the Xbox is already being manufactured. According to IGN's source, assembly of the next-generation Xbox hardware started recently at the Austin, Texas branch of a company called Flextronics, the same company that assembles 360 hardware.
According to the report, Flextronics first created a testing group dedicated to comprehensive marketing, software, and hardware tests of the next Xbox. With that finished, Flextronics started building the next generation of Xbox. Microsoft is probably manufacturing development kits, consoles game designers use to develop games.
If this turns out to be true, don't think it means that the Xbox 720 (or whatever they end up calling it) will be available soon. Microsoft has already said it's not showing a new console at E3 2012, so it might be a while until we even hear anything about it.
Neither Microsoft and Flextronic commented on the rumors.