I have a significant number of LCD panels from old laptops, so I was disappointed when I saw that the r-pi didn't have an lvds output. Given the fact that we don't (and probably won't) have access to the gpu docs or drivers source code, I have doubts on just how straightforward it will be to use the onboard DSI connector.
So I made this today, it's an hdmi to lvds adapter: http://g3nius.org/hdmi-to-lvds.tar.gz
This is an early board design, there are a couple things I haven't worked out, so it will definitely change before I have it produced. I wanted to share this sooner rather than later. It would be great if anyone with electronics knowledge could review the schematics. I made this in an afternoon so more pair of eyes won't hurt.
It works using 2 ICs, the first one is a hdmi receiver and the other one is an LVDS transmitter. between them there is 3 x 8-bit color channels, or 24 bit total. The LVDS transmitter has several modes, the one I used will be compatible with 18-bit displays aswell as [some] 24 bit displays. According to the datasheet, it will be compatible with only a minority of 24-bit capable displays but this is fine since I mostly have 18-bit LCD displays available.
The current BOM should cost in the area of 25$ for a single unit. The main IC on there is the ADV7611 which cost 15$ at digikey. It may be possible to sample the other IC from Texas Instrument and save 7$.