Canon doesn't really seem to have a set routine for using S and X. They do have the 1Ds which differentiates it from the 1D as being full frame. But I don't know why they chose an "s".
I've always been under the assumption that the S indicates a higher end model. Although most would argue that the 1D serves a fundamentally different purpose (sports) than the 1Ds series (landscape, studio), one cannot argue about the price hike for the 1Ds.
The same goes for Canon's camcorders. Note the XL1 and XL1s, XHA1 and XHA1s, and the XLH1a (no HD-SDI, genlock, or timecode) and XLH1s.