This is a common problem. From the details above, I assume you applied to derm and prelim last year, and matched only prelim, which is what you're completing now.
As has been mentioned before, it's hard to get a derm spot from your position. You need to make yourself more competitive for derm -- hopefully you did so from March to June of your MS IV, since a prelim year is not really going to make you stand out nor offer much time for research, etc. The one thing you could try to do is to get known as much as possible by your local derm program (assuming that there is one).
Presumably, you have applied for derm programs in this year's ERAS process. One of three things happen -- you get no interviews, and hence you know you have no chance of getting derm; you get a few interviews, and know there is some chance but very less than certain, or you get lots of interviews and it seems likely you'll match to derm.
In the former, you then need to figure out what you want to do. If finishing IM is is, then you would start looking for open PGY-2's to start July 2012. Those usually start appearing in Dec/Jan, which is "convenient" since that's when you'll know you have no derm interviews.
Plan A is seeing if your home program has a spot, assuming you'd want to stay there.
Plan B is trying to find a PGY2 elsewhere. There is no centralized list of spots -- it's all hard work / word of mouth. Your IM PD might be able to help you. As mentioned above, staying at your current program is sometimes the easiest to arrange but totally depends on a spot being available.
Plan C is applying into the match. Note that you'll be late for this in Dec/Jan. There are two reasons for applying into the IM match -- 1) you could match somewhere, or 2) programs could review your application, realize that you could be a PGY-2, have a PGY-2 spot open that they are not advertising, and consider filling it with you. If you match into a PGY-1, you don't "get credit" for your PGY-1 and skip ahead -- you've filled a slot that is 12 months of PGY-1 work, and you'd be repeating your PGY-1. That's why finding an open PGY-2 is much better.
If you get derm interviews, the process gets trickier. Now you need to wait until mid March before you know if you match or not. Obviously, if you match, you're all set (you'd have one year off between the end of your prelim and derm, since you'd be in the match for a July 2013 derm spot -- but you could do anything with that year). But if you don't match, you are now quite late to start looking for open PGY-2 spots. It still is certainly possible, but can be a rush, options might be much more limited, and this could impact upon your ability to get Heme/Onc in the future.
So, you're at a tough crossroads. Derm is what you want to do. Trying one more time is possible, but could make finding a PGY-2 medicine spot more difficult. Looking for both derm and medicine at the same time is possible, but if you get a medicine PGY-2 offer you're often stuck making a tough choice -- taking the IM spot and giving up on derm, or letting the IM spot go and hoping for the best.