The only game companies doing DX10 titles are the ones MS paid an enormous amount of money to. If the rest want to stay in business, they will chose to stick with DX9 for quite some time.
There are some nice features in DX10, but to do a DX10 only title would be suicide for a game company until DX10 is on all computers. Simply meaning, it will be years before there is a chance of that.