A truly "open console" is an oxymoron. What consoles bring to the table, for developers and users, is a standardized platform - a fixed set of well-understood hardware and software. For developers this means less time testing and less worksforme bug reports. For users this means guaranteed compatibility and a base level of stability and quality.
What both parties give up to the platform owner for these benefits is control. For developers, they agree to be bound by approved APIs, and give up some revenue from the product. For users, they agree to only install licensed and supported software.
One could talk about "open consoles" in the 70s and 80s, when the difference in price and power between a "gaming computer" and a "real computer" was enormous. But today, it's not meaningful. What defines a console platform is the lack of openness, in exchange for those benefits. Right now we even have platforms running the gamut of tradeoffs - PC on one end, Wii/PS3/Xbox 360 retail on the other. On the more open platforms, like the Android, we can see the compatibility and user support problems of the PC emerging.
But the value of each platform in that continuum is based on how much or how little control it takes away from you in exchange for simplicity of development/use - and with so many platforms, consumers and developers finally have the ability to vote with their code and dollars as to how much openness they want. Just don't be too surprised if, for the mass market, it's less open than PC enthusiasts are used to.
There is another, wholly useless, way to think about "open consoles", and that's in terms of hardware fetishism. (And it's what I see from a lot of people interested in the GP2X/Pandora/etc., as well as PS3 and DS homebrew.) Playing around with novel low-level hardware is fun. But it's really a secondary characteristic of what defines a console, driven by the desire to drive down component costs and size and power consumption, and to increase platform lock-in, and unfettered by the need for Win32-on-x86 compatibility. If this is why you want to do "console development", you're probably better off getting an Arduino or something.
I really don't mean to disparage low-level programming. I personally enjoy it, as a software developer. But if that's what you mean by "open console", you're really romanticizing what consoles are, and ignoring the reality of why they're still around after two decades of high-spec low-cost general-purpose personal computing devices.