The September Raspbmc update has been pushed out and unfortunately it has broken boblight support.
This release introduced a new 3.10 Linux kernel which has changed the way SPI is working.
Right now there is no fix, so I suggest if you haven’t upgraded yet, don’t!
If you’ve already upgraded then you’ll just have to sit tight until the kernel gets fixed/modified.
If you have auto updates enabled and you don’t want to update, then pull out the ethernet cable, boot into Raspbmc,, disable auto updates and then reboot with the ethernet plugged in.
I’ll update this post when a fix or a workaround is available.
Update: Sam has fixed the issue, see this post for details: http://forum.stmlabs.com/showthread.php?tid=2143&pid=83344#pid83344