First of all - tenure means little in most academic medicine departments. Tenure is important for PhD's, since it allows for job security when funding is low. As an MD/PhD, if you do clinical medicine and not 100% lab, you will always have a job. Your clinical income will support you if the NIH well starts to run dry. In fact, many academic medical centers have clinical and tenure tracts now. A person on a clinical tract still gets promoted based on teaching, service, etc and can become Assoc Prof or even Prof - just not tenured (a title of Clinical Associate Professor for example). Also, in academics, moving from one institution to another is quite common - when this happens, just because you were tenured at place A doesn't mean you'll be tenured at place B. It goes through the promotions committee just like internal candidates. You may be able to use tenureship for negotiating your contract - but there is no guarantee. Because of this, at my institution, many people enter the clinical tract.
I am finishing my residency soon, and as an MD/PhD surgeon, have accepted an academic position. I am not concerned about tenure, nor do I think that most MD/PhDs should. There is little that comes with a title in the clinical realm - people just care that you're a good physician or surgeon and if so, they will send patients to you. On the bench side, the basic scientist are often into their own thing, and yes, they do hold you to their standards when it comes to grant success, publications, etc when it comes time for promotion in a tenure tract.
Personally, just do good medicine and science, get along with everyone and you'll be fine.