I'm about to start trying an 18 mile commute on a mtb so put on a pair of Conti Grand Prix. I went for the folding version as I presumed lighter was better, a lot of the thinner 26" tyres seemed to be wire bead only.
Trial run at the weekend and I quite enjoyed how fast I could spin along on the grey stuff. Always seemed to be in the higher gears. And I felt surprisingly tired afterwards, considering how easy the riding had seemed.
Only thing was a bit tricky getting them on the rims, more so than mtb tyres. They have some punture resistance so hopefully won't have to patch up during a commute. A lot of rims minimum tyre width is 1.5", I might have been lucky as I used really old (10+ years) when I think mtb tyres were skinnier. Got the conti slimline 26" tubes to go with the tyres.
Oh, and you'll be surprised how stupid 26" road tyres look on a mtb!
http://www.chainreactioncycles.com/Models.aspx?ModelID=39338