Do you think it's a good idea to enroll even though there's a chance I might still get in? I guess it'll make a good update. Could I just take a semester of courses you think?
You'll have to balance your odds of an acceptance, vs none, vs waitlist and decide how much risk you want to take:
1)If you wait until June and don't have an improved application on a few levels, and get no acceptance, then there's no point in reapplying in June 2011.
2) If you are put on a waitlist, then communicating improvements in the application can make a difference for this cycle. If it doesn't get you off the waitlist, you are ready to reapply in June 2011 with an improved application.
3) If you engage in new coursework in January, and get off the waitlist early, due to perhaps update letters about ECs, the first year of med school will still be easier due to pre-exposure to coursework you'll be taking. So upper-level science classes won't be wasted, but they will have incurred expense. Less stress in the first year may well be worth it.
4) If you sign up for January classes and get an acceptance before classes begin, you can cancel the registration and get most of the money back (probably, but check out the policy).
5) As to whether one semester would be sufficient: what was your cGPA and BCPM for each of the last two semesters you spent in college. If either was low, then I suggest you consider taking more postbac classes next fall to more solidly establish that you can reliably produce great grades. These can be reported via update letter at he end of fall semester.
6) Does doing all this guarantee you'll get an acceptance somewhere? No, unfortunately you could go to all this work and have your application sabotaged by a bad LOR, Primary Statement that doesn't have the right tone, or a poor interview. It's for you to judge what level of effort is worthwhile toward a nonguaranteed future.