there are a few downsides to doing a transitional year internship instead of a prelim medicine or surgery internship.
first let's assume you get into a transitional year internship (which are more competitive for the reason that they usually, but not always, offer more elective time). now let's say that for whatever reason, when you apply for ophtho next fall, the outcome is that you don't match. you'll be finishing up your transitional year internship in spring 2009, but that's only a one-year program which you can't extend. what will you do the following year (2009-10)...research? well, most transitional year programs are at community hospitals that don't have academic ophthalmology programs for you to do research at. so, you'll have to look for research programs on your own, places that are willing to PAY you for your year of research. it's pretty tough to land a paid research fellowship at a well-known place that's going to pay you. it's do-able, but fairly tough. especially if they know you only plan to stay for a year...by the time they train you and you get to the point that you can really make a contribution to the lab, you'll want to leave...what's in it for them?
now say instead you do a prelim medicine year. let's again assume you don't match into ophtho in spring 2009, unfortunately. well, in spring 2009, you can go to your prelim med/surg program director and say, "look i didn't match, but i'd like to stay on for an extra year or two and finish my medicine residency", and you can re-apply for ophtho after that. typically this is possible, from what i've heard from several friends that have been in your exact situation. so, you reapply during your 2nd yr of medicine residency. and if you still don't make it, you can do your 3rd yr of medicine and reassess...perhaps by that point you'll decide you want to be a rich cardiologist or gastroenterologist instead, who knows...or maybe you'll decide to reapply for ophtho again. many people do an internal medicine residency in the US before applying for the specialty they ultimately end up practicing (ophtho, derm, etc). additionally, the training you get in a medicine residency will be valuable to you later in your career if you eventually end up becoming an ophthalmologist.
in internal medicine residency programs, as opposed to transitional programs, there is a higher chance of affiliation with a university where you can get your research going and get to know the ophtho faculty while you are in your internship year. true, prelim medicine programs generally have less elective time, but there several that offer plenty of elective time, especially if you're willing to go to smaller cities. PM me and i can tell you about the internship program i'm at right now (no ophtho program at this university, but there are 2 of them fairly close by).
so bottom line: i'd say apply to prelim medicine programs (or surgery if you're a real sadist or have some interest in it) at university hospitals where there is an ophtho department that is known to have good research and frequent publications. this will take a bunch of homework on your part, but i think it'll be most sensible in your particular situation, where matching even next yr can't be guaranteed. again, just my opinion, but someone needs to tell it to you straight so you can make an informed decision.
indy