As much as multi-touch devices have enabled new forms of music creation and performance, they are still lacking in one thing that traditional, acoustic instruments have: tactile feedback. However, Dutch designer Samuel Verburg of Tweetonig created a solution to this problem with TunaKnobs; rotary knobs that will attach to any capacitive surface (Wired UK has a great write-up on the project).
The video on the project's Kickstarter page humorously shows the problem that musicians and DJs have with many multi-touch music apps: they're somewhat clumsy to interact with during a performance.
Tuna Knobs are not the first example of this idea. Flip Visnjic has an article on The Creative Applications Network about a similar project from designers at TEAGUE. In my opinion, Visnjic is spot on when he says "I think a lot of people fail to acknowledge that the future are NOT touch screen devices but those that combine both the physical and touch input...". Interestingly enough, the only comment on the article is from someone saying how much better these knobs would be, opposed to a virtual pan knob in an audio app.
Most multi-touch devices only enable what Bret Victor calls 'picture under glass' interaction. In other words, while these devices are capable of allowing complex gestural interactions, we're generally only manipulating images on a glass screen. Our fingers only have the feeling of touching glass. Tuna Knobs seems to provide one niche-specific use for this problem: instead of turning an image of a knob with one finger, you're turning an actual, physical rotary knob in the same manner as if you were using an analog synthesizer.
Given that Tuna Knobs reached their Kickstarter goal in 21 hours, it's safe to say that there is a demand for additional modes of interaction in consumer computing devices. My prediction is that more and more work will be done to extend and augment the experience of multi-touch based computing, but (for now) it will continue to come from designers such as Verbung, not consumer electronics manufacturers.