Theres a few on Skye, im going my self on the 14th of Feb for the week. Theres the Glen Sligachan route, which is supposed to be quiet good. It was in an MBR a few years ago.
http://www.mountainbikescotland.com/rou ... achan.html
Then theres the Quiraing loop in the north of skye, not very long but lots of technical single track, as well as the old man of Storr, which is the same, but from walking it a few times it very technical. The other one I've found is Idrigill Point, Skye which starts in Dunvegan in North West Skye.
http://www.offroadadventures-online.com/rr051.html
Hope they help, as I say I have been to skye twice before so if you want any other info just let me know!
Matt.