What is not standard about the USB ports on the Xbox? You can even hook up a regular old USB keyboard to it (makes debugging so much easier, by just adding some keyboard listeners to your code and let them fire debug stuff), it's the only device except for my PC that will charge my MP3 player (USB power handshake thing) , and inside the plastic shell lies a regular laptop SATA drive, with a funky connector to connect it to the Xbox (I've broken the thing open because my drive likes to spin down after initial boot, giving me a E68, but a quick disconnect-connect fixes that). That the drive is signed by microsoft with a file on the first couple of sectors is just DRM/anti-piracy/money grabbing, but you can rip it out, format it and use it as a normal drive, nothing abnormal about it.
If you're really so gung-ho for open, I don't see why you would be interested in a PS3, with all the rampant "shove it up your ass" anti-modding updates sony has been going on for the last year or so, or the fact that you'd have to re-encode your MKV's to watch on the damn thing.
The perfect open system is a Windows PC (Linux gaming is still a sad affair, even with Wine) with a beefy videocard, a x360 controller, all hooked up to a nice TV. The Games For Windows thing makes it almost into an Xbox (can't tell the difference between Just Cause 2 PC or Xbox, even the tooltips give you the correct icons0, you get better graphics and you can do whatever the hell you want with it. A cheap dualcore system with a 5770 goes for not that much money these days, and can keep up at full HD resolutions with ease. Also gives you access to every codec you will ever need, as well as multitasking, free multiplayer gaming, home entertainment system and whatever you cook up yourself in whatever programming language that you prefer.
PS: Slight hate on the PS3 for all the "fsck linux" attitude, but not intentional as a flame post
On-Topic: I hope they release some sort of (un)official extension for XNA with kinect integration soon, do that and I will buy the thing in a trifle, just to do some hacking with it. I'd guess the thing could also be used at parties: no flying wii-motes to hit expensive TV's :D.