Currently Being ModeratedJul 14, 2012 5:32 PM (in response to Dockelektro)
I had this very same problem tonight on an iPad (first generation) with iOS 5.1.1.
I quitted and reopened the iTunes app several times but the Purchased button never showed up.
Then, half an hour after I noticed the problem, I opened the iTunes app one more time and saw the Purchased button in the bottom navigation bar!
I'm still seeing some strangeness, though, like the position of the button, that's at the far right of the navigation bar and not just before the Downloads button as it should be. It seems somehow broken, too, since the screen that should show me the songs I purchased that aren't already on my iPad reports two items in the left column and says that all available music has already been downloaded on the right one.
Perhaps occasionally missing Purchased buttons are the symptom of a temporary iTunes in the Cloud service issue. It could even affect only some specific accounts, it has not to be a general one.