Picking case for your box is probably most individual decision in whole process of building your Home Theater PC. Choose one that you like the most because it will stand just beneath your TV – you don't want any cheap plastic rubbish down there. If you have enough room in your TV shelf you should by all means pick full sized ATX case able to accept PCI cards of full size, because it will give you much wider choice when picking components later. Case should be in desktop format (not tower) for obvious reasons. Pay attention that it has at least one front USB port, because you will want to plug memory sticks, cameras and other devices, and back panel of your case will be deeply inaccessibly beneath your TV.
More enthusiastic among you may reach for even more powerful cases with front LCD display. Although I didn't find that setup useful enough to justify high prices of such cases, some of you may disagree. However, with proper setup, that small screen may give you enough information to operate your HTPC without turning on main screen (large LCD TV, probably). Imagine just listening for quiet music in dark room and you'll know what I'm dreaming about :). Although, if you choose to go that way, just make sure that backlight on such screen can be turned of programatically - you would be surprised how much light even power led can provide in complete darkness, not to mention that not-so-small LCD displays that manufacturers are putting in modern HTPC cases. And sometimes you will want just darkness in your living room, so make sure LCD backlight can be turned off.
After many days googling around, my final choice for case was SilverStone LC17 and except for the high price, I have nothing to complain about it.
