Some aspects of evolution are virtually settled. Common ancestry, for instance. But even there the details are still being sorted out - it was not so long ago that it was thought that whales were descended from mesonychids, but now we know that they are more closely related to hippos. And there are still some holdouts against the idea that birds are descended from dinosaurs so that is not entirely settled yet. And there's still a lot of research going on on the "Cambrian Explosion" (although we now know that it is far less "explosive" than a simple look at the fossil record might suggest).
On another area the whole relationship between evolution and developmental biology is a major area of research and we can expect that the theory will change to accomodate what is learned there.
So evolution is tentative in the sense that it can and will be changed as necessary to reflect our knowledge. However there are things we can have a very high degree of confidence in - and one of them is that although the theory will change, evolution is here to stay. I cannot say how the theory of evolution will change in the next 20 or 50 years, but I have no doubt that there still will be a theory of evolution and there will still be no serious scientiifc challenge to evolution - simply disputes within evolution, as there are now.
Loudmouth has already answered, but my understanding is that while there are still arguments to the contrary which have not been decisively answered, the weight of evidence is clearly in favour of dinosaurian ancestry.