You've got to love the Meet the Team videos. Seriously, it's like a law or something. Valve have been lovingly crafting them since Team Fortress 2 came out in Oct 2007 and each has been a blast and a technical improvement upon the last. Each player has their favourite: personally mine is Meet the Sniper simply for its last line, though Meet the Spy is clearly the most detailed work.
Anyway, the point is Kotaku have a few quotes from Valve's writer Erik Wolpow and desinger on TF2 Robin Walker that you should read. Among them are some insights on the origins of Meet the Team, statement of desire for a TF2 movie or short and a reitteration of plans for a TF2 comic (which seemed to pass me by the first time).
With Meet the Spy, Valve have shown what they can do with three minutes twenty seconds. To be honest, I believe even a relatively short film at 90 minutes would be pushing their creative ability and I know they don't have the time for it. To make it viable, Valve would either have to hand it over to someone like Pixar, who have plenty of experience and resources to make full length digital animation, or create a whole new branch of their company to handle the project.
So a full length movie of Team Fortress 2 is pretty much out of the question. But a ten minute short is absolutely plausible. Even better for Valve, it could be a source of revenue. Just sell it on Steam and iTunes, maybe to specialist short film TV channels. Wouldn't even have to charge people much for it, given the huge fan base Valve have. Yeah, so keep your fingers crossed for hot RED on BLU action.