Brief instructions posted my blog on how to actually add the LCD/SD we've been promising for so long.
This assumes the reader is capable of interpreting schematics only - left to their own devices the schematic is sufficient to allow board layout in Cadsoft Eagle straight-away as well
As you already assumed making it work with the firmware was more difficult, so feel free to benefit from my preconfigured Repetier Firmware supporting this schematic's Encoder, LCD, and SD card support. Download provided on the link below
I'm not going to continue bringing this version to market, as Marlin now comes with Graphical LCD support - we'll be focussing efforts on getting that ported to Repetier, but again adding an I2C component into the mix. That would be more marketable.