WORK IN PROGRESS, PLEASE AMEND
As I can't find a decent overview of how to get XBMC working on the Raspberry Pi without resorting to downloading a dedicated image, this page should give everyone a hand getting set up (that is, if I am successful...)
Ways of installing XBMC
Getting XBMC from the repos (doesn't work in Wheezy) Building XBMC on the raspi (takes a long time) Building XBMC on a Windows machine (Qemu)
So far I haven't got it running...
Building XBMC from source