Someone over on Reddit posted a patch to Luke's latest BFGminer which provides a command line option to turn on block withholding attacks.

The post is in the "I'm selfish mining at ghashio" thread.  Here's the link they posted for convenience:  http://pastebin.com/d1ptqDFv

I can think of several incentives to use such an attack:

• To prevent a pool like GHash from cornering the mining market (and thus causing prices to fall) by negatively impacting their luck.  This should providing incentive for miners to switch to another pool
• To attack pools other than one's own in order to entice their users to come to your own pool

The second incentive seems like it would only work for individuals/groups in direct control of a large amount of hashing power -- such as GHash/CEX.

Perhaps this will turn out to be just the sort of tool the Bitcoin community needs to keep mining pools from gaining too much market share?  Or perhaps it will usher in a new stage of mining wars.  Either way, it seems like it has the potential to alter the balance of power by giving the mining community some power to punish misbehaving pools.