I am using the latest nightly build and can confirm the 1/25 limit on a 600D. I cant explain why, maybe someone else can provide more detail. Anyone?
I have found that in the ML EXPO menu and using the TV/AV arrow key shortcuts in liveview, it wont go below 1/25 in Manual photo mode.
WORKAROUND:
Use the scroll wheel to drop below 1/25 s. This is the default canon method of changing Shutter speed.
Perhaps this should be moved to bug report, unless I too am missing something and this limit is there for a reason. If not, then we should be able to drop shutter speed as needed in ML Expo and LV av/tv shortcuts, right?