For my current series - yes, I have a concrete plan for what the ending will be :]
In general though, I'm not someone who comes up with the ending from the get go and 'work backwards' or 'fill in the gaps'; my idea for how my series will end develops alongside my ideas for other scenes and story directions. If I have a clear-cut idea of the ending, I generally also have an idea of how I'll get there. Even then, sometimes I'll change my mind about the ending, and sometimes I have multiple 'ending candidates' which I will choose the ending from depending on what path I feel I want to take the story through.
For one series I'm planning, it was initially supposed to be an episodic thing that goes on for as long as I have something to add to it, so I wasn't even planning on having an overall ending (of course I planned to have endings for smaller episode and arc plots though :P). But eventually it developed a 'myth arc' with its own ending, so that's probably the closest thing to an ending that series will have (I still think I might add 'bonus episodes' after the 'myth arc' concludes, sort of like anime OVAs)
Basically I have a sense of direction, but nothing's nailed down until the full story is on the page in some form