I've always seen this as a big no no but I've seen a few videos recently where people have done just that - waxed their windscreen with standard car wax (not a polish but a carnuba type wax). The idea seems to be that you clean the glass thoroughly first (maybe using an all purpose cleaner) and then apply a thin coating of car wax. It is said that, contrary to popular opinion, it doesn't streak, means you need your wipers less and makes them quieter.
This goes against everything I've believed on this topic but has anybody else waxed their windscreen and what were the results?