A New Constitutional Convention

There was a call today for a new constitutional convention, but only on a state level. Citizens in Pennsylvania, distraught over a growing number of political scandals, are considering a constitutional convention to radically reshape the government. Interestingly, a former delegate to a previous constitutional convention in Pennsylvania (the state has had five), Bob Butera remarked that the convention could "bring people out of the woodwork who ordinarily wouldn't get their hands dirty in politics, and they will get involved in the making of history."
The convention raises a larger and more interesting issue: why not a new constitutional convention for the United States as a whole? There is no indication that the founding fathers ever intended the original Constitution to be the be all and end all of American government. We've had two hundred and twenty years to see what works (separation of powers) and what might need some tweaking (the electoral college). Why not convene a new convention and make improvements on the system?
The most obvious objection to such a process is the fractious nature of today's political class--it would be hard to imagine ardent liberals and conservatives sitting down and hammering out compromises on issues such as the second amendment and the right to privacy. However, if there is a grain of truth in Butera's words, perhaps a call for a new convention would bring out that great proportion of citizens who don't "get their hands dirty" and allow us to see how democratic we really are.