Just to make sure everyone's on the same page...
The Stream (unreleased) is for streaming [edit: and sideloading] to iDevices. Coming in the fall.
The IP STB (unnamed - unreleased) is for streaming to TVs. It has no tuners or cablecard slot. Strictly a Tivo extender. Coming in the fall, hopefully.
The Preview (available from cable partners) is a full-blown STB with a tuner and cablecard slot. It functions whether there is a Tivo DVR present or not. It can pull streams from a Tivo, because it's basically a Tivo without a hard drive, but it's not the same thing as the IP box.