My Raven with ceramic rims and XTR brakes also had a very squealy front brake at first. You can toe in the pads a bit with XTR, but obviously can't do too much because the pads are quite thin in the first place. I tried toeing (and even heeling), and couldn't get rid of the noise.
Thorn apparently like to fit normal blocks to ceramic rims because they give 'better modulation'. After talking to the nice man at Madgetts in Diss, I bought and fitted some Shimano ceramic pads which are a harder compound, and the squeal disappeared. I know what Robin means about the 'modulation' thing, the normal blocks are nice and progressive and probably ultimately more grippy. But the ceramic pads are much quieter, and I can trim the levers to increase braking force if I feel the need (not happened yet), so I'll be staying with ceramic pads from now on.