Is the X51 a case of "you got your peanut butter in my chocolate and it's awesome" or more of "you got your oil in my water and it doesn't really work"?
The "Steam Box" may or may not be a thing that exists, but even if it doesn't there are PCs whose purpose it is to vie for a position next to any consoles you may own, not stand in opposition to them. The Alienware X51, which the rumour seems to indicate might be some sort of early spec for the Steam Box, is certainly a PC that wants to make you think it's a console, but of course that begs the question: does it need to be?
Putting aside whether or not you actually like the company or the products they produce, this seems to be a case where they really want to sell the PC as a sort of console alternative: hook it up to your tv and play what you want. It's still a computer of course, and is capable of serving as your main rig if that's your desire, but the way it seems to be pitched is that it won't be your main computer, but rather your gaming computer. I'm not really any sort of computer expert, but the people who reviewed it at The Verge seem to think it was solid for what it was meant for.
The thing about all of this though is that I can't help but feel that it's a really niche market they're targeting. I'm not one of those people that thinks that console and pc gaming are on different sides of a fence and never shall the two meet, but this still seems like a sort of reaching effort. I'll admit that I've been heavily considering getting a better PC lately, and certain you could qualify what I'd want to build as a gaming machine since part of the plan is that I want it to be able to do so effectively and hopefully with some longevity in terms of what it can handle and what standards it can handle it at. Anyways, digressing to the point, even though I undoubtedly want a gaming PC, the design and aesthetic of the X51 just seems sort of off to me, it's not something that I think I'd turn to for my average day to day computing needs, nor is it something that I'd only hook up to the tv to play whatever I've happened to download off Steam lately.
I know it might sound stupid, but it all tends to boil down to "it can do it, but should it?" realm of reasoning for me. For me PC gaming is indicative of certain experiences and is better at certain gamestyles (like FPS, obviously) whereas when I boot up the PS3 I can play Devil May Cry, or Katamari Damacy, or whatever else. A PC is also something that I'm just as fine using for everything else a PC does, so having one only and ostensibly for gaming seems like a strange idea at best, and a ludicrous waste at worst. Just like my PS3 can browse the Internet, but does that mean that I should us it as my main web browser? Hell no. That would just be crazy.
I'm not sure if this Steam Box, again assuming that it's actually happening, will face these same challenges. It seems to be an uphill battle, but perhaps not one without hope, and certainly not without purpose.