I kind of see the idea of a personal mission statement as being what do you see as your purpose in life? Like Paul said, it has nothing to do with your current tasks on your todo list, your occupation, job, or what your goals are.
For me, that would be:
My purpose in life is to help people because helping people makes me happy.
Now, see, that has nothing to do with any specific occupation or job, but defining it can help me set goals, seek an occupation in line with it, and fulfill in my heart what I feel is my destiny. Knowing this, I won't seek a job or spouse that isolates me and prevents me from interacting with people I can help and makes it easier to ensure I stay away from and don't do things contrary to it.
And Paul is right, you can't define this unless you really look deep inside yourself and discover who you are.
I tend to avoid Vision Statements or long term life goals, and I definitely won't put them in writing. I have learned the hard way that doing so leads me down the path to serious depression, because my life rarely ever goes in the direction that I have planned, and looking back and seeing where I had hoped to be by now and where I actually am, makes me feel like a worthless failure. I'd rather not do that because it tends to spiral out of control and can be quite dangerous to my mental health. (this was a big contributor to my GTD related breakdown a few years ago) I can't look forward without looking back, and looking back makes me revisit things I'd rather forget and plans I made that never came to be.
It has taken me quite awhile to come to terms with the idea that it is ok to live life without a master plan, long term goals, or a big picture view of things. Some people are best going with the flow and enjoying the adventure. Some of us are a different type of traveler, better suited to just going wherever the road takes us, rather than having a destination in mind, using maps, planning routes, and having a travel itinerary.
I'll wonder about it, maybe dream a little, but I'll always deliberately leave it very fuzzy. I am better off that way. It helps me get through today with my sanity intact.