Stormtroopers aren't all clones.
If they were all clones, they'd have had the blasphemous dubbing over that Boba recieved on the DVDs.
The fact that Boba now has Jango's voice (and ACCENT! WTF?) and the Stormtroopers do not, indicate that the Stormtroopers are something else.
Also, from watching the movies, I always assumed the clones are gradually killed off and replaced by human conscripts. The way The Clone Wars is fleetingly mentioned in A New Hope suggests the clones are part of a bygone era, and the fact clones are never mentioned again also suggests they are a thing of the past. By A New Hope, the Imperial Academy is in full swing and thats how new Stormtroopers are brought in - along with what Lucas said about officers getting preference for cloning.
I have always imagined that a really cool scene in a prequel would have been to have the Emperors statue being erected while thousands of young men are waiting in line by a building and walking out as Stormtroopers.
I am hoping for a scene like this in Ep III at least for the sake of ending this issue or adding some clarity to it.