Let me preface this by saying that I am starting the Tufts MBS program this fall.....
So here is my thinking/opinion/observations about whether or not to apply this year for medical school...
If you have already applied to medical school and were not accepted then there was something in your application that did not impress the adcom, this could be have been MCAT, grades or volunteering, etc. If it was volunteering/MCAT and you have improved significantly in those areas then I would say go ahead and apply this year. But if you are in a post-bac/SMP to improve your grades then...
1. the primary application comes out very soon and you should get it in ASAP. We all know that the earlier you get it in the better chances you have. This means that you wouldn't have even started your SMP/post-bac program yet. Nothing significant would have changed on your primary application by the time you have to submit it. You could write in your personal statement that you are enrolled in a post-bac/SMP but without grades the adcom has no idea if you will be successful.
2. you won't have 1st semester grades until most of the secondary deadlines have passed. This means that you would have to send something in later with an updated transcript. Most people I know who have been successful in getting interviews had already been invited and had completed their interviews before your update would be sent in (early Jan.). I think this would put you at a disadvantage since the adcom would have to wait until they get the transcript to make a decision, probably pushing your interview back to late Feb/early March. It is more likely that you would interviewing for a waitlist spot at this point.
3. Most medical schools encourage a 2nd application but frown or even won't allow a 3rd application. So unless you are absolutely sure you will get in the 2nd time you may be shooting yourself in the foot. Say you do apply this year for your second time and you aren't successful but then you do awesome in your post-bac/SMP, what happens? The schools you can apply to a 3rd time are more limited and might not be the school you want to go to.
If you were to wait another year, you would have a full year's worth of post-bac/SMP grades and more time in a volunteer position. This would also give you more time to get meaningful recs from professors since they won't have to write one only knowing you for a few weeks.
I know you really want to go to med school (as do I) but waiting that extra year may make all the difference!
The only exception to this are schools that have conditional acceptances (like BU and Georgetown) that pretty much say if you do well you will get in. But even that is not a guarantee. I went to the BU open house and was told by the director that the % of students who get in to med school when they apply the same year as they do the SMP is only 5% but it jumps to 60% for those who wait that extra year. Just something to think about.
As for me, I am completely happy waiting that extra year if it means getting in and not having to deal with all the rejection all over again!
Good Luck!