Hmmm... so I have my raspberry pi now. I put Raspbmc RC3 on my SD card using the installer and got only one red LED when turning it on. I then put debian on the SD from the raspberry pi website and it booted fine. I think that may also have updated the firmware? I then put RC3 back on the SD again and it booted, it went through all of the installation and then went to reboot. Afterwards it came up with an error:
mmc0: error -84 whilst initialising SD card
mmc0: error -84 whilst initialising SD card
Kernel panic - not syncing: No init found
I have found the error -84 is fairly common, I then turned it off and on again and it doesn't give the -84 error but still gives the kernel panic.
I am not sure why

The SD card is a Samsung 8GB class 10 that is on the compatibility list and so it should be okay. I don't think its the power, I've tried two supplies (one rated at 700mA and the other at 1000mA).
Any ideas?