yeah you can read medias directly from an HDD already and you can transfer files with some soft from the xbox HDD to the computer....
but for movies I don't see the point you can't keep more than 12 hours once you started them..... it's more the DLC and the full games from XBLA and oldies from XB...
I don't see it happening for hack issues I guess...

@neoRatt
you mean I can download movies on my PC and save it to my external harddrive and see it through xbox360?

 yeap depends the format of the video though... even burned DVDs with movies on it lol :) it's more the other way the issue... stocking stuff from XBL on a external HDD...

Yes, I do have my DVD movie library ripped to DivX and stored on the Western Digital Passport USB powered 250 GB hard drive.  When I want to play them through 360 I just connect the drive and choose the media source to the drive.

Unfortunately, this is what takes up the most space formost people.  20GB is ample for game saves, and most gamers don't stream the movies onto the Xbox.

Honestly, I expect thisfeature to be included in the Spring update--MS loves money, but we'll only spend so much of it in the Marketplace if we only have a limited space to put stuff.  I actually bought a 500-gb external drive, assuming it would be compatible with the 360.  Lo and behold it isn't, yet it's compatible with the PS3, which has FAR more space than I'll ever need...