This is a legacy thing for MobileMe Gallery which is still lingering in there. When MobileMe finally bites the dust, and Apple replace the Gallery with the rumored new iCloud Gallery (not to be confused with PhotoStream) I imagine they will issue new updates for both iPhoto & Aperture which will replace one service with the other.
Until then, bare with it. Oh and maybe try logging into MobileMe pref pane in system preferences - even though you have transitioned already - it may stop the nagging prompt.