I kind of hope both will stay around. The competition between them is good and it does push the envelope in design of the API's.
Microsoft will never adopt OpenGL, at least not directly. It may slide in over time, but it won't be one of those, "Hey, let's forget about DX and go to OpenGL", kind of things.
Of course,..DX10 may have some surprises for a lot of people. 'Surprises' may be too weak of a word. The future looks neat.