This video demos a free software project, XStremeTouch, that lets you add a multi-touch JazzMutant Lemur control interface to the Yamaha XS workstation.
It also supports iPhone and iPod touch interaction, via an application called TouchOSC.
At this point, XStremeTouch is "not a finished product by any means but is an ongoing personal development and research project". Future plans include extending this to support voice editing, performance editing, step sequencing and more, not only on the XS, but on other manufacturer's equipment.
See the XStremeTouch site for more details.