I'm really enjoying my PhD but I'm worried about post doc-ing or lecturing afterwards. My department is a mixed one but my subject matter is being pushed out in favour of other ones. There is now less and less demand for my subject and less classes to teach as a result of one over-zealous department leader. I am now worried than my PhD will be a waste of time, because what I really want to do in the future is teach and there doesn't seem to be any stability in my subject area, at my university.

Does anyone else have this problem, or have any advice/suggestions for my predicament.

I'm sorry, I don't want to go into too many details about my subject area because this is, essentially, a public and traceable forum.

Thanks in advance for any replies.

Hey Argh,
well the world doesn't end at the boundaries of your department! if you are willing to move, and you should be, if you are looking at a career in academia - well then you can "simply" go to where your subject area is more prominent.
there are advantages and disadvantages to the fact that your department is changing.
+ no new graduates - less competition for you!
- in time, your dept. might get known as "weak in subject x" and thus you having graduated there/received your PhD there might be slightly devalued.
but at the end of the day, if you write a strong PhD, any place that cares about your subject will be interested in you.
for now, if you are worried about teaching experience, can't you teach anything at all? teaching experience in related subjects counts, too, you know. and can be quite valuable! and can demonstrate that you are no narrow-minded specialist who cares about your specific narrow topic and nothing else...

Is there some reason why you can't move? I can't move and I recommend making your teaching skills as broad as possible. Don't know how common this is, but as grad students we are expected to be able and willing to teach anything at first year level - across the entire faculty. I have tried it and it is challenging but doable as long as you keep one step ahead. I expect your subject will always be taught to some extent as well.

My subject is somewhat in the outer orbit of topics in my dept but I'm hoping to capitalize on the fact that I'm the only person doing it - combined with taking on whatever other teaching is thown my way.

you can always try a position in any other universities.. so as long as your work is good and not that rare in the outside world, it is no problem finding a position out there.