Previously the device had been an invite-only deal for developers, but a page appeared on the Google Store today offering up the developer kit for $512 to anyone willing to pay.
However, it swiftly disappeared again. Here’s the cached page:
It’s likely the product broke cover a little early, ahead of this year’s Google I/O which kicks off later. We’ll be there bringing you all the news from the event, including, I suspect, more Project Tango news.