I installed Democracy, the decent Videocast client program on Mac mini and am enjoying the High-Def content from the Web, like Rocketboom, Ask a Ninja and Diggnation. (Some of them might not be technically HD but the quality is good enough).
One of my fave sites to find trailers is Dave's Trailers Page, which collects links to mostly HD movie trailers on Yahoo! movies or Apple trailers page every week. But the site doesn't have an RSS feed and there's no way to track or automatically download the new trailers, etc.
So I made the feed. You can subscribe to the feed using Democracy or just default iTunes, and hook the computer to the TV. Every day you can get the High-Def (the feed is linking to 720p quality one) trailers automatically, which is very nice.
For those who are interested, It's done by 20 lines of Web::Scraper script and the latest plagger on SVN. You can change to 1080p if you want, and run it yourself to host the rss feed file. I guess you might be able to do this with Y! Pipes but scraping data is dead easy if you use Web::Scraper module, which I'll talk about in YAPC::Europe in Vienna.