Check in Console.APP, see if you are getting an error related to /var/folders. It could be that it is not at your location yet or that the servers are very busy.
You can try deleting the actual folder in var/folders/<Install folder> with the download but I would wait, since it seems to be a demand issue from my location. Similar to the Lion roll out, it is going to take some time to spin up...
On your Mac you can check for the errors.
Follow the error down and look for
7/25/12 12:37:36.526 PM App Store: FRPurchaseManager: Preflight operation for 537386512 failed with error: Error Domain=com.apple.appstore Code=0 "The product distribution file could not be verified. It may be damaged or was not signed."
<User Data Clean out here for public post>
{NSLocalizedFailureReason=Cannot create PKProduct from "file://localhost/var/folders/xxxxxxxx
Error Domain=NSURLErrorDomain Code=-1100 "The requested URL was not found on this server."
<User Data Clean out here for public post>
{NSUnderlyingError=0x7fdb488a7ab0 "The requested URL was not found on this server."
NSErrorFailingURLStringKey=file:///var/folders/0c/kp18vvb97nn2b5czzwq2yh9m0000gs /C/com.apple.appstore/537386512/preflight.pfpkg/index.sproduct, NSErrorFailingURLKey=file:///var/folders/0c/kp18vvb97nn2b5czzwq2yh9m0000gs/C/co m.apple.appstore/537386512/preflight.pfpkg/index.sproduct, NSLocalizedDescription=The requested URL was not found on this server.
"The requested URL was not found on this server."}
Just going to have to wait, demand is very heavy.