I did an MD/PhD program with the PhD in biomedical engineering. My research was related to design and testing of drug delivery devices, and I went on to do a residency in radiology. The technological aspects of biomedical engineering are particularly relevant to radiology, including device design, image processing, MRI physics. My PhD experience has been applicable to my clinical field and helped drive my research interests.
I would definitely advise you to choose an MD/PhD program that has a long track record of people doing BME. If you are blazing a trail, you are likely to run into trouble. My school had a history of many people doing BME, getting strong support from the department, and finishing in a timely fashion. At least 1 person a year did BME, and with my class there were 5 graduates in BME. Of the people I remember, here are some of the career trajectories:
Two people: Radiology residency -> fellowship -> academic medicine
Radiation oncology residency -> academic medicine
Neurology residency -> academic medicine
No residency, straight to device industry
Radiology residency -> device industry
As you can see, there are a variety of things you might do.
One thing I don't understand that well is your statement that you definitely want to go into cellular/molecular biology bench research. That sounds closer to many other fields that biomedical engineering, although, as mentioned above, tissue engineering is one possibility.
If you are interested in more info, PM me.