In the end though, is it not all about money? As far as I am concerned, grad school is something you do for fun, and which costs a lot of money. Even if you receive a minimum paycheck, you have to take the opportunity costs into consideratiosn, so five years of grad school cost several 100000$, or am I mistaken? Why then do you worry so much more about starting a startup? The differences in maximum loss seem marginal, except that for grad school the losses are guaranteed, and for startups they are not.
It does not depend on being in grad school or not whether you can do what you want, it depends on having the money - either to pay for grad school, or for toying around with your startup.
I realize I made some oversimplifications, but still...
At least where I go to grad school (Ohio State University), grad school does not cost anything. You either win a Fellowship, you teach a class, you help a professor with his research, or you do grunt work like grading papers and labs. In any case, tuition, fees, and 80% of health insurance is covered, in addition to a $1600-1850 per month stipend. That works out to a respectable $20k/yr.
If you do not get a Fellowship, you are expected to perform 20 hours worth of work per week (which you actually receive credit for), in addition to your time spent on classes and on your own independent research.
In other words, grad school is more like a low-paying job than a cost. You could make the case that you could be working a higher-paying job in that time, but then you are missing out on the flexibility of being a Ph.D., which allows the possibility of being a professor or getting hired for a pretty high income. Additionally, you have the opportunity to leave your mark on the field with your research.
Even more interesting is that Ph.D. work could lead you to your startup idea, as you unveil a new use of existing technology or a new approach altogether which leads to a money-making possibility. Furthermore, if you are exceptionally driven, you may be able to get some work done on your startup idea while attending grad school, because at least so far, I have found grad school to be significantly easier than my undergrad was, because I am taking less "real" classes.
It does not depend on being in grad school or not whether you can do what you want, it depends on having the money - either to pay for grad school, or for toying around with your startup.
I realize I made some oversimplifications, but still...