The Raspberry Pi foundation has announced that The official Raspberry Pi touchscreen display is now available. This is a ten-point touchscreen, which connects to the DSI connector and has full kernel support, after an upgrade.
Given that this has taken the foundation several years to finish, I doubt it is something that anyone could easily hack together from spare parts.
The Wikipedia Display Serial Interface page says:
The Display Serial Interface (DSI) is a specification by the Mobile Industry Processor Interface (MIPI) Alliance aimed at reducing the cost of display sub-systems in a mobile device. It is commonly targeted at LCD and similar display technologies. It defines a serial bus and a communication protocol between the host (source of the image data) and the device (destination of the image data).
At the physical layer, DSI specifies a high-speed differential signaling point-to-point serial bus. This bus includes one high speed clock lane and one or more data lanes. Each lane is carried on two wires (due to differential signaling). All lanes travel from the DSI host to the DSI device, except for the first data lane (lane 0), which is capable of a bus turnaround (BTA) operation that allows it to reverse transmission direction.
According to this forum thread DSI screens will need new GPU drivers to enable DSI output:
if you manage to use the dsi connector (i.e. kidnap some bc engineer and let him produce a new gpu-binary for your dsi-connected screen) you could in fact have a working dual display setup...
As such, although official DSI screens are now available, and there is kernel support, it is unlikely that that such support would extend to arbitrary other implementations of DSI screens. Given what the wikipedia page says about the protocol, DSI is hardly a smart way of doing things these days (the protocol is complicated by the need to support displays without on-board memory) and it is possible that any given GPU binary may only work with a limited set of specifically supported devices, rather than supporting any arbitrary DSI display.