If you put a "rear" motorcycle tyre on the front then you have to mount it backwards. i.e., the rotation marks will point the wrong way. This is because the main force on the tyre is braking - not accelerating as it is on the rear.
There is a thread on this subject somewhere from a year or two back and I (think I) posted a link to a page from a manufacturer's site saying the same thing. I was also at my local dealer the other day and he commented that a lot of GoldWing guys were doing that and getting phenomenal mileage from them. Many of them were also running Dark Side.
So the bottom line is - it shouldn't be a problem. However, rear tyres often have a flatter profile and may be a bit heavier. So if you are trying to beat the sport bikes around tight corners it may not be the tyre you want.