Trying to stir up the message board a little bit here too, but I've been working on writing ground up programs for the xbox, entirely in MASM.
Following some of the examples from http://www.xbdev.net/non_xdk/nonxdk.php
I was able to successfully build up a small library of routines to print text to the screen, change the front LED colors, reboot and power off the machine. The only thing standing in my way of actually creating a working application is initializing the USB controllers to accept input. No small task, but it's not impossible either.
The machine is very much PC like, and I found that with enough patience and tweaking, virtually all of my existing MASM source can be recycled into a xbox code.
If anyone is interested in any of this, let me know or post here and I'll beam you my work in progress. You will need a modded xbox to run the code from, but you do not need the XDK or any other development kits-everything is built using MASM 6.11, DOS, and a bit of external hacking on the final output .exe.
Don't start thinking you'll be able to make the next Grand Theft Auto with any of my code-it's really raw and low level at the moment, but it's an interesting hobby to mess around with if you'd like a different sort of platform to work with.