The increased precision in engines nowadays have decreased the length and need of a break-in period. Still, I believe the owner's manual states a break-in for the first thousand miles, unless that has changed recently. I would take it easy for the first thousand miles or so.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record