According to FUD Zilla, AMD will release the Radeon HD 4870 X2 in the last week of August. The site claims something is causing a delay but they're not sure what it is:
Partners should get the evaluation samples in a week's time, but something will delay mass production to late August and it's not the driver.
All GDDR5 boards with 2GB memory will be built by ATI but there is a chance that ATI will let its partners to make some of the GDDR3 boards.
The board comes with interconnection bridge chip but we’ve heard that this time PCIe bridge supports PCIe 2.0 while the one on 3870 X2 could only support PCIe 1.1 interface.