I have had a raspberry pi type B for a few weeks now and one problem that I have is on the SD card which can get stressed and corrupt XBMC data. I was looking for a solution tonight and eventually stepped on it reading a few pages from the wiki. Since I still want to use the SD card, I just changed /flash/cmdline.cfg to contain this:
ip=dhcp boot=/dev/mmcblk0p1 disk=NFS=192.168.0.66:/media/xbmc
Of course, you will need to change the NFS server and mount point as they are likely to be different. Worth noting that I rsync’d the content of /dev/mmcblk0p2 (/storage in OpenELEC) so I would not have to scan my whole collection again.
There is also a full procedure to netboot your Raspberry Pi but this did not appeal to me, details there.