treetrunk,
many of the secondary applications that i filled out had two separate sections: 1) why md/phd or what are your goals as a physician scientist? and 2) describe your research experience. i found that treating these topics separately (at least, at first) was easier than immediately trying to combine them into one long md/phd essay. with these two shorter essays in hand i was then able to cut/paste and revise my work into the cohesive md/phd statement that some secondary applications require. perhaps approaching it from along those lines will help you to overcome writer's block because, as newquagmire correctly pointed out, time really is of the essence.
specifics:
for my research description, i clearly stated the problem, the hypothesis and the research plan/methodology. then i described the results and the various possible conclusions given those results, including any potential issues or nuances for a particular conclusion. finally, i suggested other work that could be done and/or how the results fit in to the larger problem in science.
for why md/phd, i just described how i developed an interest in becoming a physician scientist and tried to tie it in to specific examples from my research experience. then, rather openly, i stated my long-term career goals and how md/phd was an important step in achieving them.
you can probably see that there are several areas of contact between the two topics which facilitate a fairly seemless integration into the larger md/phd statement.
well, feel free to pm me a draft when you have it done. but give me at least two days to get back to you.
good luck,
aaron