Thunderchild, it's called electrical tape for a reason =) Vinyl is an extremely good insulator, unless he's dealing with more than 1000 volts (I hardly find this likley) there's nothing to worry about.
And yes solomar, you could just use electrical tape to isolate it, the only problem being I don't think a bearing would make a good electrical conductor, they're covered it grease which will likley insulates it somehwat, if you're transfering more than a trivial amount of power you'll end up heating the bearing up, and you'll have a whole messload of electrical noise, but it might work. Check the bearing on a meter and try testing it with your load current on the bench before you decide.
I like Mike's copper tube and spring loaded brush idea, I think I'd trust it a little more than using the bearing as a conductor, but if the bearing works it's mechanially simpler. You could investigate the possibilities of conductive grease for the bearings as well, then it's a win win situation.