Not working for me either.
Same in France . Hopefully maybe 10.8.2 is coming !
Just click on ok and the update should complete.
It says No Update Available when I click on OK, seems like a temporary glitch which should be cleared up by Apple before too long I hope.
I had two updates when I had this message, but I was able to still run the update via the MAS.
i downloaded itunes 10.7 from http://www.apple.com/itunes/
it wroked but here s my question will it be a problem if i install it manually will it be a problem for later updates via the appstore?