There's some truth to the notion that some medical schools have already admitted some of their students by this time. However, many of these students are Early Decision applicants. The remainder probably DID take the April MCAT. So, taking the April does put you at somewhat of an advantage.
However, there's ways to increase your chances if you're still taking the August administration. Firstly, apply to AMCAS early. I turned my app in 3 days after it was released online. I've been verified for a good 6 weeks now. A lot of med schools will send out secondary apps LONG before MCAT scores are in (I've gotten 17 so far). If you send them in right away, the schools need only scores.
Moral of the story... if you need to take the August test for one reason or another, no worries. There's still PLENTY of time to get interviews and all those acceptance letters!