A new build of Windows 7's first Service Pack has appeared on torrent sites which judging by the id of "6.1.7601.16537.amd64fre.win7.100327-0053" shows that it was compiled on March 27 using a 64-bit AMD processor.
As we've learned in the past from several sources, including Microsoft employees, the service pack won't be bringing anything radical to the plate, but the biggest additions will be official USB 3.0 support as well as enhanced Bluetooth and Wi-Fi stacks. However, it's yet to be determined whether or not this particular build includes these additions.
It's been said Microsoft's plans to release Windows 7 SP1 sometime during September of this year. A Beta build should be ready by June.