Locking a repeater can only be done with another repeater. This is done to prevent massively breaking contraptions by powering it other ways. Almost no one should have repeaters directly feeding into usually unusable sides of repeaters, so this shouldn't break anything like using a redstone line next to it would. They are doing minor changed to redstone for 1.4 - 1.5 is planned to have massive redstone overhaul which, although destructive to older maps, will be preferable in the long run.
Edit: I just tested locking a repeater. It not only locks it off, but can lock it on, essentially turning repeaters into a 1-way redstone torch. They give power without a signal until unlocked.
Edit2: I put the main repeater on 4-tick delay, the one attached to it 1 tick delay. Both attached to a single lever. I flicked the lever, and since the attached repeater was faster than the main repeater, it locked it to the off state before it could make the 4 ticks to turn on. This is interesting behavior... Wonder how it could be used.
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