[322.339 MH/s (~291 MH/s)] [Rej: 73/7908 (0%)]

Is the display output from one of my poclbm miners, and the (~ MH) is one of the newer, and my favorite stat that I basically look at as local luck. As I understand it, the first number is how many hashes hashed from the miners point of view. The estimate is a timed based average depending on how many shares have been accepted (maybe + rejected? Not enough to matter it seems). So sometimes its a bit higher than the actual hash rate, and sometimes lower, like now.
These 'lucks' get added up in a pools mining stats, and basically multiplies by how many you have running. If all your miners on a lucky streak, your reported mash rate in the pool stats will be more than your actual.