Colluding miners attack: Is the Bitcoin protocol compromised? A very interesting academic paper has been recently published by two researches from the Cornell University (abstract, full PDF):


  Abstract
  
  The Bitcoin cryptocurrency records its transactions in a public log
  called the blockchain. Its security rests critically on the
  distributed protocol that maintains the blockchain, run by
  participants called miners.  Conventional wisdom asserts that the
  protocol is incentive-compatible and secure against colluding minority
  groups, i.e., it incentivizes miners to follow the protocol as
  prescribed. 
  
  We show that the Bitcoin protocol is not incentive-compatible. We
  present an attack with which colluding miners obtain a revenue larger
  than their fair share.  This attack can have significant consequences
  for Bitcoin: Rational miners will prefer to join the selfish miners,
  and the colluding group will increase in size until it becomes a
  majority. At this point, the Bitcoin system ceases to be a
  decentralized currency. 
  
  Selfish mining is feasible for any group size of colluding miners. We
  propose a practical modification to the Bitcoin protocol that protects
  against selfish mining pools that command less than 1/4 of the
  resources.  This threshold is lower than the wrongly assumed 1/2
  bound, but better than the current reality where a group of any size
  can compromise the system.


The article suggests that miners who group together can earn more than their fair share, which means rational miners will eventually group together into a pool whose size is larger than half the network's computation power and control the currency. The method was termed "selfish mining" by the media.

This seems to be the first serious cryptographic exploit of the Bitcoin protocol.

Do you think this poses a real threat to the stability of the currency?
The report is not heavily disputed.

In the hours following the paper's release, several developers are discussing solutions:


http://www.mail-archive.com/bitcoin-development@lists.sourceforge.net/msg03128.html


and 


http://bitcointalk.org/index.php?topic=324413.0


There has not been any evidence that such an attack is imminent or likely even.  And remember, even an attacker with 51% cannot do much.  One risk is that a transaction could be canceled after it has confirmations, if the selfish pool is that much further ahead.