In fact I have seen a successful small rocket launched from a 100 ft high ballon platform (the physics is painful but it is possible).
Other ideas like centrifuge acceleration I know have been discussed: and I personally see potential in them (Ammers & Myers, Oxford University Press, 2006 I think: I'll try and dig out a reference #).
With equal respect I suggest you actually try to think outside the box before posting vacuous statements stating it is impossible.... ;)
EDIT: BTW you do know NASA have recently been exploring the possiblity too.... admittedly for low earth orbit sattelites (and i know that a major problem is breaking that barrier - but it is proof of an initial concept....)
The crucial data is that ballons can carry rockets to a MUCH higher altitude than planes. The reduced drag at that altitude means a lot less propellant is needed (and even a solid fuel propellant could suffice).
NASA has plans for a ballon that can carry up to a ton of equipment to a height of 33Km which is pretty good!
You've also missed the point of the airplane: 600MPH is frankyl nothing (4% of the speed required for LEO). The important thing is the lowered drag and kinetic requirements to get the payload to LEO.