JP Aerospace has an interesting concept where the entire balloon would get slowly accelerated to orbital velocity using electric/chemical hybrid propulsion: http://www.jpaerospace.com/atohandout.pdf

That could potentially be viable, as it'd save fuel over more traditional rockets by allowing for the use of high efficiency, low thrust engines which would otherwise be infeasible for an orbital rocket due to gravity losses.

Not sure though, I haven't done the math. It might be that the losses due to drag exceed what a more traditional rocket would lose to gravity.