It appears that the Radeon R9-290X won't need CrossFire bridges, and that it doesn't even have the ports.
Even after the AMD Hawaii event, a lot of information about the AMD Radeon R9-290X continues to trickle out from unofficial sources, the latest being from Donanimhaber. One thing that has come to see daylight, or rather, hasn't, is the lack of CrossFire connectors on the upcoming Radeon R9-290X graphics card. At first this might have been mistaken for being part of engineering samples, though on second thought a lot of sources are pointing to the use of "Sideport" technology.
Sideport technology is an older technology, last used in the Radeon HD 4870 X2. In other cards, both AMD and Nvidia have had to use bridge connectors to allow for enough bandwidth between the cards to operate together. Recent developments though, now allow otherwise. While the first PCIe specification, PCIe v1.0, had a bandwidth of about 4 GB/s (for a 16 lane PCIe slot), the PCIe v3.0 specification has almost 16 GB/s for the same slot. The upcoming PCIe v4.0 specification doubles that yet again. As such, the PCIe ports alone might provide enough bandwidth, rendering the use of a CrossFire bridge unnecessary.
Of course, it is very possible that this might still bring problems. For example, while most motherboards have a PCIe 3.0 x16 interface, what happens to some motherboards when more than one of these ports are populated is that a number of them (or all of them) have to share lanes, meaning that each port, or graphics card, effectively gets either eight lanes, or in some dramatic cases, just four lanes. That said, all of this is still speculation, so take it all with a grain of salt. It is very possible that AMD might not be calling it 'Sideport' technology but perhaps something else.