Study: Opportunistic mining could harm mining poolsNakamoto Ryo self-published a study on how Bitcoin mining pools are potentially vulnerable to pool-hopping by miners who hope to maximize returns.
The reason a miner might consider pool-hopping is because the longer a mining pool round takes, more shares will exist and thus each share represents a smaller and smaller slice of the reward pie. A miner might then realize a better return by switching to a pool with a relatively new round which would thus have fewer shares already contributed.
As discussed in a forum post, today this is probably a moot point until there are at least two pools with similar hashpower.
The PDF of the study is available for download without charge.
[Update: Another study addressing pool cheating was performed by Raulo. A PDF of that study is available for download without charge.]