A summary of what has been done:
prpplague was the furthest along last I heard, and has J-Taged in the open-source image into his, as well has replaced the rom in one of his.
The ROM is really a Read-Only Memory chip not flash, so it has to be physically replaced to change the ROM image. There are a couple of flash ROM chips that are compatable, but you have to either program the chip externally or connect the ROM write lines in the JuiceBox and boot it via JTag then use a utility to write an image to it. No exploits have been found to boot from SD Card, but you could in theory have a pic JTAG in a small bootloader (which you'd probably have to make one yourself).
As far as playing your own movies, there's no known way to trick the built-in player to play off SD card. The open source image/source package doesn't include the PVPPlayer. You could probably compile something like mplayer or a small MPEG player program and modify it to use the hardware buttons and play small 240x160 encoded files. (not enough processor power to do scaleing at speed)
As far as the exact pinouts of different things, from I've seen on mine, everything should be fairly easily traced from the S3C44B0X pinouts. In fact, most of the pins have nice vias (holes) close to the chip.
I may trace out the SD card adapter I have and make a full schematic for it. If don't mind paying $40 for it, K-Mart still has the MP3 kits.