We use a Pay-Per-Last-N-Shares (PPLNS) reward system, where N is set at 2*Difficulty. The basic method is this: When a block is found, payment is given to shares in a window, starting with the most recent share submitted and going backwards up to N. Shares older than this are not paid. So if difficulty is 1 million, then the most recent 2 million shares will be counted.
PPLNS does away with the concept of "rounds" and miner rewards are instead determined using a moving time window. As you mentioned on the crowdmine forums, this is similar to BTCGuild's concept of "open" and "closed" shifts. PPLNS was originally proposed by Meni Rosenfield, and Meni has gone on to compare it with other reward systems such as PPS, ESMPPS, DGM, etc...
