Yes. YiTian is a keyboard layout to input Chinese. You're right in saying "it's a different allocation of standard bopomofo symbols among the keys."
For someone like me, who has learned bopomofo phanetically as a kid, but grew up in the US learning how to type using the QWERTY keyborrd. It makes a lot of sense for me to use YiTian as an input method.
In standard bobomofo layout, ㄅ(bo), ㄆ(po), ㄇ (mo), ㄈ(fo) are 1, Q, A, Z keys respectively. This is like saying you'd have use 1, Q, A, Z key to type ABCD, and 2, W, S, X keys to type EFGH, etc.
It makes no sense. Since we all grew up learning the QWERTY keyboard layout. You wouldn't want to relearn how to type if keys are layed out as ABCD, EFGH, IJK...
What makes YiTien layout work is that B key = Bo, P key = Po, M key = Mo, and F key = Fo. If you already know how to type QWERTY, you can pick up YiTian in no time.
That is why it is absurd that YiTian layout completly dissapeared from 10.7. How else am I suppose to type? I don't want to learn a new keyboard layout all over again if I can avoid it. If I got that time, I'd go learn a new music instrument or something.