Originally, you couldn't accept your own answer at all, for the following reason:
Although it’s fine to ask and answer your own question — this is specifically encouraged in the faq — you’ll have to rely on the community to upvote your answer and validate it as correct. You, sir (or madam), are biased. Of course your answer to your own question will be the best possible answer. You wrote the darn thing!
After some arm twisting, Jeff relented and they changed the code to allow one to accept their own answer, however, the community still had to vote that up as the "best" answer.
If you write the play, you can give the first billing to whomever you want, but if you want to give yourself first billing you're going to have to prove to your underwriters (the community) that you deserve it.
You should read the whole blog post, including the comments, to better understand why this is in place.
http://blog.stackoverflow.com/2008/11/why-cant-i-accept-my-own-answer/ <-- LOTS of discussion on the sort order issue
And the follow-up where he relents:
Practially speaking, though, it's not an issue. Few questions have more than 4-6 answers, and anyone interested in the problem is going to read all the answers until they find the one that solves their problem.
Perhaps if you gave some examples of questions where it's obvious that not having the author's accepted answer at the top renders the question damaged, or leads people trying to solve the same problem astray.