I have this problem as well. Can't install/update any application via App Store.
My mac is connected through wifi to a wireless router that is connected through a vodafone usb 3G connection. Everything works well: HTTP, SSH, and even P2P protocols such as Bittorrent!
So, I don't have any reason to think that my ISP provider is the source of the problem.
This is more likely a bug in AppStore.
With Console.app I get the following messages from storeagent:
4/16/12 9:58:21 PM | App Store[1969] | Calling purchase callback |
4/16/12 9:58:21 PM | storeagent[1961] | CANCEL 2: -[DownloadQueue _notifyClientsAboutDownloadsChange:forSource:] |
4/16/12 9:58:42 PM | storeagent[1961] | *** Assertion failure in -[SoftwareInstallOperation _startInstall], /SourceCache/Pisa/Pisa-60.8/Source/iTunes Protocol/Built Into App/SoftwareInstallOperation.m:307 |
4/16/12 9:58:42 PM | storeagent[1961] | error Error Domain=SoftwareInstallOperation Code=158 UserInfo=0x1010c5a90 "NSInternalInconsistencyException" |
4/16/12 9:58:42 PM | storeagent[1961] | localException [PKProduct productByLoadingProductAtURL:file://localhost/Users/Luis/Library/Application%20 Support/AppStore/505089833/mzm.pycdmiam.pkg] failed (null) |
4/16/12 9:58:44 PM | storeagent[1961] | CANCEL 2: -[DownloadQueue _notifyClientsAboutDownloadsChange:forSource:] |
Please help!