what i did...
coming from a 1.1.3 (mar 08 build)
Downloaded the 1.2.3 nightly from
http://builds.magiclantern.fm/#/copied all and pasted to sd card, the camera doesn't start
so i went to canon's website and downloaded the 1.2.3 firmware placed the .fir in the sd card with the old 1.1.3 ml.
camera starts and updated to 1.2.3.. camera won't start with sd card inside, works normally without sd card.
copied bleeding edge items over to sd card.
camera starts
update using the .fir provided (the 40kb one)
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for anyone who wants to try i guess this is some rough instructions. follow at own risk
1. download the canon 1.2.3 update fir from the official website
http://www.usa.canon.com/cusa/professional/products/professional_cameras/digital_slr_cameras/eos_5d_mark_iii#DriversAndSoftware2. place the 5D300123.fir in to the sd/card that still contains the 1.1.3 ml
3. use canon's menu to update from 1.1.3 to 1.2.3 - ensure u have enough battery. this takes about 2-3 mins
4. the camera won't start once done, remove battery and wipe card.
5. download the nightly (link above) and place them on the card.
6. the camera should start now and use canon's menu again to update using a1ex's 5D3_123.fir
7. you should get this screen

8. you're done!

camera works perfectly now..gonna test the functions