If Windows 8's new UI turns out to be even slightly successful then it is a safe bet that monitors and computers generally will come with touch built-in, and so while not useful for gaming itself, a metro style Steam client application may become a real possibility some years down the line.
To that end, as an enthusiast of UI and UX I decided to experiment with the concept of a Steam Metro UI. Please note that these shots (especially of the store) are very early WIP mockups.
Obviously there are a number of design decisions that would have to be made, as well as lots of end-user testing for this to be a viable UI for steam, but for an experiment I think it's an interesting start and welcome constructive feedback!
Non-Metro Steam UI Concepts: