Here is another Scott tidbit for you to consider, when I first started college I was sure I was going to major in physics and work for NASA. I still love physics, and science in general, but as I soon found out calculus sucks. So I became an english major and haven't looked back since.
It looks like there is some evidence that gravity may not be so constant:
One possibility is that invisible, so-called dark matter is holding the spacecraft back. Some cosmologists believe that dark matter exists because only 10% of the expected mass of the universe has been found. If 90% of the universe's mass and energy is invisible, maybe it could exert gravitational pull on spacecraft.
Another possibility, even more fanciful, is that invisible dimensions of space are tugging at the Pioneers. This idea has its origin in string theory, an idea that suggests we are surrounded by far more than the three dimensions we know about. Some versions of string theory suggest there may be as many as 11 dimensions, most of which are curled up and hidden from us.
As with dark matter, no hard evidence has been found proving the existence of vibrating strings far tinier than the smallest known particles.
Brings me back to 4th grade when we had to stand in front of the class and give a speech; we got to choose our topic. My topic of choice? Gravity waves.