Radiopopper just announced that the RP Cube will be available starting Monday, May 3 ($29.95). I fully expect them to sell out their initial stock.
About the Radiopopper system:
The RP Cube is part of the Radiopopper JrX Studio and JrX Basic systems. The JrX Studio system allows radio control of flashes in manual mode. For radio triggering of flashes without the ability to adjust the flash output remotely, there's the JrX Basic system, which functions like the cactus radio trigger, except it's supposed to be more reliable.
Until now, the JrX Studio system could only be used to trigger strobes with a data port (the Alien Bees, White Lightning, and Zeus strobes). The JrX Studio system could not be used with hot shoe flashes (JrX Basic, on the other hand, works with pretty much any flash with a miniphone jack). But with the RP Cube, the JrX Studio system can now be used with hot shoe flashes. The hot shoe flash just has to be a dedicated TTL flash for your camera. The RP Cube is available for Canon and Nikon.
For remote flash control in both TTL and manual mode, Radiopopper offers the PX system. Radiopopper PX is available for Canon and Nikon. PX transmitters work by converting the optical wireless TTL signal from the hotshoe flash, commander (Canon ST-E2 or Nikon SU-800), or the Nikon popup flash into radio, which the PX receiver converts back into an optical signal for the remote slave flash. Radiopopper originally had not been designed to work with Canon popup flashes but now that the Canon 7D offers built-in commander capability like Nikon's, in theory, the PX may be used with the 7D popup flash.
JrX and PX Interoperability:
A PX transmitter can communicate with a JrX receiver, except that the PX will only be able to control flash output manually for that flash. Similarly, a JrX Studio transmitter can communicate with a PX receiver, except that again, flash output control will be limited to manual mode.
Update: on ebay, someone is selling their own version of the RP Cube for Nikon flashes, for the same price (seller: bufo1955). The seller cautions however that it doesn't work with the SB-900. The seller also sells a cheaper alternative for the SB-800 and other flashes with a TTL jack: cable only (but it will not work with the SB-600 or SB-900). The seller notes that you don't need a Nikon body with either unit, but you do need to use a Nikon flash.