Floating launchpad

Another wacky idea here, but I have a lot of idle time to think on my mindless job. I have no idea if this idea is even plausible, but I happened upon an issue of Popular Science about the use of lighter than air craft that were massive enough to use as tramsportation of enormous payloads, and that set me to thinking about ... you guessed it...space colonization. If a payload could be launched from a floating platform miles above the Earth's surface, could that significantly cut launch costs in the long run? Of course getting the payload to the launchsite would still be a pain, but perhaps one could use yet again another large zeppelin to ferry the payloads to it. A simple elevator cable would be nice, but impractical, as not only would it have to be miles long, but strong enough to lift the payload and support its own considerable weight.