One thing that puzzles me about the Chromecast compared to other Google/Android devices - why has Google locked it down so much and not released a bootloader unlocker? Is it for publisher/DRM-type reasons (given that the Chromecast is primarily for playing content on your TV)?

Its certainly for DRM reasons. HBO, Netflix, and a host of other services support casting

Yeah, almost certainly this. Same reason Sony locked down the Dash (the Sony branded chumby device) -- it was essentially required by netflix (in turn because it was required by netflix's content partners).

Still, it would have been nice of Google to put in a developer mode for the Chromecast that basically wipes all the proprietary bits and turns it into a generic linux box. Though, granted, at the price they are selling the hardware for that may not be the best business move... and in any case, there are other options for that in the form of all the many linux/Android-on-a-stick devices (like all the Rockchip based ones), but having one you can easily buy at retail in the US would be a nice option.

I find it amusing that Netflix is one of the main reasons we have both locked-down devices like the Chromecast and DRM in w3c standards. It all boils down to the content providers sticking their fingers into tech, but Netflix's popularity and leadership in the space is probably the direct driver of both of these.

People who can do this level of hardware hacking amaze me.

This years Defcon CTF has some RF hacking. An FPGA running an MSP430 was one of the flags. That's some cool low level stuff.