I just receive my Raspberry Pi today! All has gone very well; I was able to install Arch on my SD card (Kingston SDHC 8GB), expand the partition etc., and boot everything up. (In Arch, I did need to remove /boot/config.txt to get the HDMI to display to my DVI monitor.)
I installed xorg with xfce4, which after a bit of a wait does open up and works quite well. I wanted to see how much of the RAM was being used by xfce (and whether I'd rather go to a different, unfamiliar session), and found something a little perplexing in the output of top:
Note the Mem: line– it lists the total memory available as 125004 kB, not the 256 MB I was expecting… is this normal? If so, where is the other half of the RAM?