Each week Ross Rubin contributes Switched On, a column about technology, multimedia, and digital entertainment:

Last month's D: All Things Digital conference saw the launch of two platforms from well-known technology companies -- Microsoft's Surface, the coffee-table PC that can be used to sell coffee tables, and Palm's Foleo. the big-screened mobile companion for pocket devices from the company that popularized pocket companions for big-screened devices.

Both products rely on recent iterations of well-tred operating systems. The heart of Surface is simply a Vista PC, whereas the Foleo is based on Linux. But their usage models could hardly be more different. Surface is a large tabletop computer environment reminiscent of the cocktail arcade tables of the 80s but which is actually filled with infrared cameras and a projector -- a new application of rear-projection TV technology.

Foleo, with its small clamshell form factor, eschews any kind of touch-screen manipulation, instead introducing a scroll bar to facilitate moving through long Web pages and lists of e-mails. In contrast, at least for its initial kiosk-like deployments, Surface will take advantage of new applications that use its direct manipulation and recognition of physical objects. However, both products illustrate the challenges that companies have in trying to introduce a "fourth screen" to compete for consumer attention beyond the three screens of television, the PC, and the cell phone.