You have to put it in EVERY pages Page_Preinit event. It will not work in a master page.
Kind of a pain but better than other alternatives like browser caps etc which I could never get working.
Protected Sub Page_PreInit(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreInit
If InStr(Request.ServerVariables("http_user_agent"), "Safari") Then
Page.ClientTarget = "uplevel"
End If
End Sub