Thank you all for your responses...they're invaluable!
Ok, so it seems it wouldn't be the best idea to try and squeeze it in. But I am 23 which makes me think I'm too old to be taking it slow...or is that not the case?
I'm confused about the timeline for MCAT/Applying/Acceptance. Say I decide to go through with it, on the Fall '10 plan. I would take all the pre-reqs this coming academic year (Fall '08 + Spring '09) and then take the MCAT in... April? Or August? (are those the only times it's given?)
After the MCAT, how long do you wait to apply? Say I take the MCAT in April of '09...that would be a good time to take it for admission Fall '10?
Also, what is BCPM gpa? And wouldn't any extra classes I take after my BS in ME not change my gpa anyway? I know that I've only got a 3.3, but in engineering land that is a solid number (I was on dean's list most semesters)...do medical schools take into account that it was a 3.3 in engineering? Or do they just look at it being a 3.3 in general?
Okay, here's the things I would do:
1. MCAT: Take it so that you get your scores back by June 1. The MCAT website has the "predicted score release dates" on there somewhere. I would make sure that I took all the bio, most of the organic chem, and have a solid understand of microbio and biochem. Also allow for time to review your gen chem and physics. Take the practice tests and see how you do. If you do really well, take the MCAT sooner. If you don't do so well, take the MCAT closer to April / May. The MCAT is offerred on the computer multiple times a year. Pay attention to sign up dates for the MCAT (several months prior to test date) because some sites in some months fill up quickly and you'll end up going to BFE to take the test.
2. Submit your AMCAS primary application as early as possible. Yes, it is possible to get admitted to medical school if you submit late, but the application process sucks enough already and you want to give yourself every advantage. That means you start writing your personal statement sometime around beginning of April so that its ready and reviewed by people who have good grammar and writing skills (engineers are known for their lack of writing ability, myself included).
3. Buy the MSAR, its available on the AMCAS website as well. Its got great info on every school.
4. Start thinking
right now about who you want to write you letters of recommendation. You'll need at least one science professor (I don't know that engineering counts for science at all schools), so target one this upcomming year and work at developing a strong relationship with that teacher. Sometimes schools (UCSD) require a letter of rec from a humanities professor, or something of the like. Look at the websites for your top schools, look at their requirements for letters, and make sure you've got those solid relationships. Make sure you ask them to write the letters to allow enough time for it to be submitted to your college letter service or Interfolio, bearing in mind many profs go on vacation over the summer. I would ask them in May, with gentle reminders every now and then. Letters of rec are sent with the secondaries.
5. If you submit your primary in June, you should be verified in a matter of days (mine took a month since it was in late Aug). You'll get your first round of secondaries in a few weeks to a month from schools that don't screen. Shortly thereafter you'll get the schools that do screan. Fill these out really well, have someone with good grammar and writing skills review it, and get them in. You will be sick to death of writing, but push through it. With any luck, you'll get your first interview invites towards the end of summer.
6. Apply broadly, especially if you live in California. I would apply to at least 20 schools (use MSAR as a guide). Apply to some reach schools, some schools in your range, and some slightly below your range.
7. BCPM means take the GPA of all your physics, math, gen chem, ochem, and bio (including micro bio, etc). Engineering does not count into BCPM. And I realize that a 3.3 in mechanical engineering is awesome, however do not count on admissions committees giving it a second thought. Some may care, some may not, and in some cases a computer will be the one doing an initial GPA/MCAT numbers screen.
8. Get clinical and medically related experience. If your top school is a research institution (Stanford, UCSF, UC Davis, etc, etc) then find a way to get some research experience (all the while developing relationships with the doctor you shadow, the researcher you work for, for the eventual letter of recommendation).
9. Make sure you have money saved for this process. For example, primaries cost $150 for the first school, $30 for each school you add after that. Secondaries range from $60-$130 each. Then there's travel expenses to interviews, the cost of taking the MCAT is ~$200, if you take a review course for the MCAT that's more $.
Oh, and I'm 30, soooooooo, yeah, you're young......
Good luck

. Engineers rock, you'll do fine.