This may sound a bit harsh but I'm just trying to be honest.
I agree with brickmanli.
I applied in the 2000-20001 application cycle before the whole AMCAS online debacle and it really hurt me when I took the August MCAT. In addition I submitted to AMCAS the first week of August. I interviewed late as a result because most schools decided not to review my file at all until the scores came out in October. By the time I interviewed, in January/February, most schools that I applied to had already filled up their classes. I ended up not getting in and took this last application year off.
Since it looks like AMCAS has its sh** together this year (cross your fingers :wink: ) applying late this year will end up hurting you a lot more. Since this past application cycle was rather atypical due to the problems with AMCAS, it should not be used to gauge the effects of taking the MCAT in August and applying late since most applications this past year were delayed anyway for one reason or another. Many schools that do not automatically send out secondaries to applicants did so anyway in order to bypass the AMCAS submission "bottleneck." With AMCAS working smoothly this year this probably won't happen again.
The vast majority of medical schools recommend that you take the April MCAT. I think they must know *something*. They are more aware of general trends of acceptance and I would place more weight on what they say than on anectdotal evidence some posters have offered on this thread regarding friends "A","B", and "C" that took the August MCAT, applied late, and had "no problem" getting in.
With that said, don't fret too much about taking the August MCAT but understand that you will be competing for fewer seats at schools that have rolling admissions. By the time you get your scores, many schools will have already sent out acceptances. Some even delay sending you a secondary until your scores are in as was the case with me (this really hurt BTW). In addition to school with rolling admissions, I would look for schools with non-rolling admisssions where the playing field would be more or less equal with April MCAT'ers.
Hope this clears things up a bit. Good Luck.