Some questions/points to consider:
What's the consistent throughput of your broadband connection? DSL or Cable? It affects how quickly you can start watching a movie while it's downloading.
Most movie download sites have different resolutions based on your access speed. With your HDTV you'll likely want the better quality versions. Some online video sites (Amazon) stream as fast as 2.5MBPS, most are in the 300-1500KBPS range.
What's the speed/memory of your laptop? You'll need a pretty fast processor to be able to cleanly stream Standard Def. movies now. I don't know what will be needed when someone starts streaming Hi-Def. Although somebody probably is by now, I just haven't seen it.
Better laptops have HDMI output options now. And your choice of Blu-Ray or HD-DVD player/burner.
But consider a Media Center PC (horizontal desktop) for upgradeability. Face it you're never going to need
less hard drive space once you start building a library. So adding additional storage will be easier. Especially if you want to store/play DVD's also.
There's some pretty slick ones that come with
remotes and Windows Vista. Most of the download sites use a propietary interface that runs on Windows Media Player. iTunes is the exception but you won't like the quality of iTunes movies on your 47" screen.
Otoh if you want to use a laptop you can buy long HDMI cables to have the laptop near you, but cables much over 16' sell for over $200.
I personally haven't seen a great (
legal) movie site yet. I think the best one so far is Amazon's.
Amazon.com Unbox video downloads: TV shows, movies... Here's a link to other options:
Which Movie Download Site Is Best? The article was written last January so there may be better options now.
There's also the new service Vudu, but it's based on their propietary set top box.
With a fast connection and newer hardware I'd say yes. Netflix even started offering streaming movie downloads recently so there must be some demand. Anyone with a 5-10MBPS connection probably wouldn't be able to tell what the source is, slower than that you probably shouldn't use your connection to websurf simultaneously.