OK, so it's not made by Dell, but the tip about the portable SideShow companion device turned out to be true. And it even looks kind of similar to the iRiver Clix, which is how I pictured it originally. In fact, though, the companion device is really thin, and it actually looks almost exactly like the Meizu Mini Player. (Unfortunately, I haven't been able to get my hands on an image of the companion device, so that's the Mini Player you see at right.) The gadget I saw on the show floor at CES 2007 is made by MSI--though other manufacturers could make them in the future--and it's about the size of a credit card, albeit one that is a quarter-inch thick.
Unfortunately, the Microsoft rep whom I accosted didn't have a lot of information regarding the hardware, except to confirm my suspicion that it has onboard flash memory, meaning it can function as an MP3 player and a photo viewer (and probably a video player as well). Of course, the main purpose of the companion device is to act in conjunction with SideShow, giving you access to your RSS feeds, calendar, e-mail, and so on--no matter where you are. Plus, if you're within wireless range of your computer, you can use the unit to control Windows Media Player and PowerPoint presentations.
Update: It looks like there is indeed more than one manufacturer working on the SideShow companion devices. Engadget spotted one with Dell branding out on the CES show floor. The MSI one that I saw looks exactly like the device pictured at left, except with "MSI" printed where the "Dell" is. And see how similar it looks to the Mini Player pictured above? I wonder if there was some collaboration--or "borrowing"--of design there.