What really annoys me on IMDB and the likes is the amount of ratings a film gets even when it's still a year or so out from being released.
The only potential problem I see with this is when people see pre release/premiere screenings of films if you're in the industry or have worked on a feature... Then you wouldn't be able to mark it as seen... Dunno surely there is a nicer way around this.
And regarding Andrew's comment... I think if it were just an email notification or something with all the settings for it controlled in the settings page, then there'd be no extra clutter in the design ;)
And it'd be cool to be notified when it's released on DVD also... So if a film is on my watchlist, comes out in the cinema, but I don't get a chance to watch it and haven't marked it as seen yet, then another notification upon DVD release could be cray...