Only if you have permission to display the trailer. If someone illegally uploads the trailer to another site, and you then link to it, you can technically be held responsible.
i would never upload anything that im not sure on my server/hosting just do it through all those great 3rd party video uploading sites veoh, megavideo, youtube, and etc...
Hosting it on another site doesn't protect you because linking to it could still be considered contributory infringement. Again technically you can't post copyrighted trailers without permission. That said it is extremely unlikely that the MPAA or whoever is going to sue you simply because you are promoting the video which in theory should lead to more people buying tickets. It is however still a remote possibility.
Wrong. Just linking is to material not hosted on your site can be contributory infringement. It's what shut down Napster - which didn't host any content. "one who, with knowledge of the infringing activity, induces, causes or materially contributes to the infringing conduct of another, may be held liable as a "contributory infringer"."
Unless you own the rights to the clips or explicit permissions to it, it would be illegal to do what you plan to do. Of course, there is nothing wrong is you provide a hyperlink to the official website's main page which contains the video clip.
Upload it to a third party site or more preferably, find the trailer thats already on a third party site. If a movie studio wants you to remove the video, they will send a takedown notice, at which point you can remove the video and not be in legal hot waters.