Dragonclaw,
To the question you ask mkirkpatrik about the speed of gravity, I can tell you that most physicists believe the speed of gravity is equal to that of light, and there are recent experiments (just do google search) that show this is probably true. By the way, that speed is 300,000 km/s (in Spain we measure in km, maybe you do in miles?).
Now, the thing about E=mc^2 and F=ma is that for Einstein's equatin, m is variable, whiles for Newton's m is constant. So we can't put together or connect the equations in any manner, because their variables and constants are different. We can't, simply. Becuase we end up to absurd equations, such as the one you give or one I derive here: m=E/c^2=F/a so if c^2=a then Energy=Force. I hope you now understand why we can't think of them both together.
By the way, yesterday I was surfing around royal's society web and I find out a very interesting debate done this year about Einstein and Newton. They debated who was a better genius, who was a better scientist, who contributed more positivelly to humanit, and who help contributed more taking in accout the scientific knowledge at each's time. It's long but quite interesting, I give you here a link:
http://www.royalsociety.co.uk/page.asp?tip=1&id=3879