First off, you may be overly optimistic that if you start applying for jobs in September, you'll automatically get swooped up for a job in less than a month. Now, you might be in a field where this is the case. However, you'd hate to be in the position that you put off job hunting when you shouldn't have. Also keep in mind the summer months often means different hiring staff are off on vacation, so it may take longer to hire than in most otther times of the year.

You can always, in your cover letter, state briefly but clearly that your availability will start in October. That way, no one's jerked around.