The Raspberry Pi (RPi) does exist, but while waiting for it to be delivered here are some notes to make it easier to begin porting Xinu to the platform. (Work in Progress)
The RPi uses a fairly standard USB micro-B connector for power (only the power pins are physically present). It needs 700 mA at 5 V, so most computers or cell phone chargers should work. Embedded Linux source
The shipped RPi board doesn't come with a serial port attached (no surprise there). But it does have an expansion header that includes pin outs for a serial port (and more). The details can be found on the Embedded Linux RPi page. You will probably just want to solder on the full 2x13 pin header then deal with the outputs later incase you want to take advantage of the other pins.
You will want at least a Class 6 or above SD card (lower class cards have been known to have read failures). I (Michael) ordered two Kingston 8 GB Class 4 SDHC memory cards from Amazon. 8 GB is probably excessive, but they only cost ~$7 each, and if you intend to have a full Linux install available it might not be a bad idea to go for > 4 GB.
Xinu is, of course, the target operating system for the RPi platform. However, there are some steps to take before we get there.
Sadly the RPi does not come with a standard bootloader. Instead it simply boots off the SD card (and only the SD card) when power is applied, so a straight from the network boot is not an option. However, SD cards are low cost so you should be able to install a bootloader on the SD card that enables a network boot. It may "taint" the environment, but it shouldn't be horrible or worse than a typical boot loader.
It looks like the grub bootloader should be able to support network booting Grub PXE network boot. This is probably the most viable option, but other options may exist and work better for the RPi.